I've given much thought to your photos.
And to avoid being chopped up into green waste!
Tell me what the soil content is that it is growing in?
What I would do: take it out of the pot and remove as much of the old dirt as possible. Next I would re-pot using a richer potting mix like Miracle Grow 2 parts and 1 part perlite. Then I'd let it readjust and give it ample amounts of liquid fertilizer every 10 days between normal watering schedule.
I'm going to say it's more hungry than your other plumeria.
I wouldn't worry about the middle number on the chart, just use regular Miracle Grow for all plants.
Could be it needs a richer soil is my guess.

I hope others come to give ideas here, but I feel this is what I'd do for this situation.

Stuart

I'm not familiar with that variety's characteristics. The vendor is well known and to best of my recollection has a good reputation on this forum.

Have you used any kind of sprays or treatments? How about your fertilizing and watering routine? Soil Conditions? I would assume if you have 25 other plants (assuming plumerias) you have a good system but thought I would still pose the questions. When you transplanted into the 7 gallon how did the root ball look? The tips look healthy from the pictures. I am not so sure that damp and cool nights could be a major factor.

I would do two things...Make sure its not a something like a fungus or insect and start making some adjustments. Maybe relocate it in the yard to a warmer more sunny location or change your fertilizing routine to same quantity but in smaller, more frequent applications...monitor, adjust, and monitor some more. You can figure it out. Don't give up on it.

I am not sure the soil mix is the issue. Last year when I planted most up to 7 gallons, and in prior years, my preferred mix was roughly half and half Miracle Gro potting soil and pumice with a handful of slow release fertilizer thrown in and a couple of cups of worm castings mixed in on the top. Followed up with a half cup of fertilizer, usually something like a 6-24-24 once every 4-6 weeks, and occasional weak higher nitrogen fertilizer solution mixed with liquid seaweed. But I can ramp up the fertilizing, see what happens.

Thanks for the input, I am curious to see if anyone else has thoughts on this.

How about increasing watering by a marginal amount? See what happens. The plant is mature enough to survive a few days of extra moisture.

I see similar leaves when my trees are too dry.