The fertilizer is fine, as long as your watering and fertilizing habits work together to ensure A) the total level of salts in the soil solution doesn't rise above levels that significantly limit the plant's ability to take up water, and B) you're flushing the soil when you water to ensure the ratio of nutrients in the soil don't become skewed to the degree that excesses of one nutrient limits uptake of 1 or more other nutrients.

It looks like over-watering to me. I suggest you start using a tell.

Using a 'tell'

Over-watering saps vitality and is one of the most
common plant assassins, so learning to avoid it is worth the small
effort. Plants make and store their own energy source –
photosynthate - (sugar/glucose). Functioning roots need energy to
drive their metabolic processes, and in order to get it, they use
oxygen to burn (oxidize) their food. From this, we can see that
terrestrial plants need air (oxygen) in the soil to drive root
function. Many off-the-shelf soils hold too much water and not enough
air to support good root health, which is a prerequisite to a healthy
plant. Watering in small sips leads to a build-up of dissolved solids
(salts) in the soil, which limits a plant's ability to absorb water –
so watering in sips simply moves us to the other horn of a dilemma.
It creates another problem that requires resolution. Better, would be
to simply adopt a soil that drains well enough to allow watering to
beyond the saturation point, so we're flushing the soil of
accumulating dissolved solids whenever we water; this, w/o the plant
being forced to pay a tax in the form of reduced vitality, due to
prolong periods of soil saturation. Sometimes, though, that's not a
course we can immediately steer, which makes controlling how often we
water a very important factor.

In many cases, we can judge whether or not a
planting needs watering by hefting the pot. This is especially true
if the pot is made from light material, like plastic, but doesn't
work (as) well when the pot is made from heavier material, like clay,
or when the size/weight of the pot precludes grabbing it with one
hand to judge its weight and gauge the need for water.

Fingers stuck an inch or two into the soil work ok
for shallow pots, but not for deep pots. Deep pots might have 3 or
more inches of soil that feels totally dry, while the lower several
inches of the soil is 100% saturated. Obviously, the lack of oxygen
in the root zone situation can wreak havoc with root health and
cause the loss of a very notable measure of your plant's potential.
Inexpensive watering meters don't even measure moisture levels, they
measure electrical conductivity. Clean the tip and insert it into a
cup of distilled water and witness the fact it reads 'DRY'.

One of the most reliable methods of checking a
planting's need for water is using a 'tell'. You can use a bamboo
skewer in a pinch, but a wooden dowel rod of about 5/16” (75-85mm)
would work better. They usually come 48” (120cm) long and can
usually be cut in half and serve as a pair. Sharpen all 4 ends in a
pencil sharpener and slightly blunt the tip so it's about the
diameter of the head on a straight pin. Push the wooden tell deep
into the soil. Don't worry, it won't harm the root system. If the
plant is quite root-bound, you might need to try several places until
you find one where you can push it all the way to the pot's bottom.
Leave it a few seconds, then withdraw it and inspect the tip for
moisture. For most plantings, withhold water until the tell comes out
dry or nearly so. If you see signs of wilting, adjust the interval
between waterings so drought stress isn't a recurring issue.

If your plant is healthy, you might consider setting part of a day aside to repot it (if it's in need). It might also be a candidate for defoliation. Healthy Ficus defoliated in June (if you live in the N Hemisphere) can be pruned and strategically defoliated to stimulate a new flush of pristine foliage.

Al

I would've thought it was sunburn

The symptoms of sunburn are nearly always more severe toward the center of the leaf and less severe at leaf margins, except in the unusual case where only the occasional margin is exposed. This is because high temps cause a higher level of excitement in molecules of the plant's photosynthesizing machinery, which produces oxidizing compounds (hydrogen peroxide being one) as the molecules return to a normal state. Thinner tissues at leaf margins lose heat to surrounding air more effectively than the thicker tissues at the leaf center, so they produce less oxidase which = less in the way of sunburn at the margins.

Al

Thank you! That's good to know

And in this case it couldn't be that the leaf edges are more prone to sunburn because the plant is a variegated type (because of the difference in color - and whatever is the cause of this)?

( i heard that these variegated types are more prone to sunburn than the non-variegated varieties)

Thanks

Al, thanks so much for the no-nonsense information and in-depth explanations on how you draw conclusions. They enable me to understand and learn to correctly diagnose problems in plants, rather than rely blindly on expert advice, which may not always be readily available whenever I need it.

Thank YOU for your kind words, Adela.

Al

Thank you very much for your thorough answer, Al. I can't read 1 sentence by you without learning something new :D

Good! I try hard to provide reliable information that others can used to get more from their growing experience. It's rewarding to be able to think the info is being put to good use!

Al

So sorry for the delay in responding! Al is right, definitely not sunburn. The plant is placed about 7’ from a wall of large south facing windows, and had been absolutely fine in that light for about a month before the issues began. I did recently add a layer of soil to cover some of the plant’s was exposed root ball; at the same time we got a period of VERY humid weather. I suspect those together are what’s choking out the roots. I’ll try to repot to a shallower pot this week and report back!

Update: The leaves have become decreasingly turgid, indicating issues absorbing water as Al suggested. The plant dropped the two bottom (and most damaged) leaves. Since then I've repotted it in a smaller pot with a coarse mix (1/4 cactus mix, 1/4 potting mix, 1/4 perlite, 1/4 coarse orchid bark) and it's already looking better. It is now standing on its own for the first time, without the aid of a bamboo rod or soil piled up over the top floating roots. Hopefully we'll see a return to its former turgidity level and increased growth on the crown soon!