..... appears the soil is to water-retentive and compacted. Roots need oxygen to function normally. If the soil doesn't hold enough oxygen or you over-water, The plant's internal water pressure drops, causing loss of turgidity and wilting. The best way to tell if the/a soil is 'completely dry' is by 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 plenty of air (oxygen) in the soil to drive root function. Many off-the-shelf soils hold too much water and not enough air to support the kind of root health most growers would like to see; and, a healthy root system is a prerequisite to a healthy plant. Watering in small sips leads to avoid over-watering leads to a residual build-up of dissolved solids (salts) in the soil from tapwater and fertilizer solutions, 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. Al

It looks like residue from insecticide or fungicide applications; or, early on, the plant was in a situation where it was watered from overhead and the water was allowed to repeatedly dry on the foliage, leaving dissolved solids residue on the foliage. It's possible too, that the same chemicals mentioned above were injected into the water used for over-head watering and were left behind on foliage as the water dried. I don't think it's cause for concern. If it interferes with the plant's ability to carry on photo synthesis, chemical messengers will inform plant central the leaves aren't carrying their weight, and the plant will automatically shed them. Too, your plant looks healthy enough that removing the affected leaves shouldn't create a scenario where the plant is using more energy than it's creating by turning the sun's energy into food ..... so you can remove them w/o need to be concerned. Remove them by snipping through the petiole (leaf stem) about half-way between where it's attached to the stem and where it transitions to the leaf blade. The stub you leave will fall off on its own in a week or two. Pulling leaves off can easily damage latent buds resting in leaf axils (angle formed by a leaf and the branch it's attached to). Al

No history - no way to tell. It looks like sunburn or a phytotoxic reaction to something applied topically to foliage. Could also be evidence of resorption, which is the first stage of leaf abscission (shedding). Again - no way to determine the cause by an image alone. Al

Rubber plants do seem to go into hibernation over winter. Mine put out zero growth for almost 7 months at temperatures b/w 0-18 C. The top leaf had stayed as a bud and then when spring hit it suddenly started sprouting! Keeping it in maximal light certainly boosts its growth though if you want larger leaves keeping it in partial shade helps.