Third season, mid August, not the first large leaf yet ??

After planting cutting and a winter inside under grow lights, the plant was rotated outside
and back inside for the second winter, after the leaves fell off. No grow lights were used
the second winter. The lower, browner section is the original cutting. The rest is the
second years growth outside. The plant had large leaves the first two seasons.
A window blind fell on the top of the original cutting the first spring and damaged the top
and knocked off a few leaves.





The whole plant photo shows the plant in mid August
the third season. The third season, it was moved outside
and fertilized. No leaves have grown larger than those
shown on the photo of the top.
They come out, turn black, new ones come back,
still small, etc. etc. Little growth of the stem noticed.



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Seems like it suffered some fertilizer burn to the roots. Ideally I would have waited for new leaves to sprout naturally first before you apply any fertilizers.

Since it is summer, try to soak that container root zone area, in water, then allow it to drain away, hopefully it flushes out the excess fertilizers. Set aside in an open part shade area to dry out..and wait patiently.

You also may have root rot. Your medium may be too heavy and it could be staying too wet.
I would unpot the whole thing which will give you a chance to inspect the roots. Then repot in a well draining medium incorporating 1/3 to 1/2 perlite.

If root rot is present, what's the best protocol? Trim it back? Just transplant everything into a drier and better draining medium?

I recently potted four (different) rooted plumeria, all same way, in well-draining medium. Each plant arrived from Hawaii with large leaves clipped, bare roots, so it took the better part of a week to get them into pots given shipping time. I sprinkled a time release organic fertilizer on each.

The tiny, but slightly larger, leaves on three of the four plants slowly turned black and eventually fell off.

The smaller claws or just emerging leaves on three of the four plants have all begun growing, some plants with new claws and leaves emerging from tips.

On the fourth plant, however, that looks somewhat like @mollydog's, the two leaves turned black and fell away, and nothing since. There's one claw that always was present, and it's not doing anything. It's been potted about 14 days now.

Sometimes, even if daytime is hot enough, the night time conditions may suddenly turn colder than it likes, with the morning dew, black tip fungus happens. At times hard to control it, it just happens, so it impedes further growth. It only takes a brief exposure for that to occur unfortunately.

Typically if I see that happen, I try to feel the entire stem, if no soggy part, then I just do a bit more watering interval for the time being, till plant recovers. Got to help it feel more warmth/heat all over, and wait patiently once more. It is still summer anyways, so it may still bounce back late in the season though it may be too short to expect blooms since it will be in recovery stage.

and I also think you may be keeping your medium too wet. When no leaves are present water VERY sparingly.

Thanks to you both. I added a ton of perlite hoping that would make the soil faster draining, but will add less water when I water next. I only watered once when I first potted the plant, but I think with too heavy a hand.

Separately, I just realized the perlite I used had fertilizer in it. I've now gotten coarse, plain perlite, which is what I thought I had before. Ugh. Maybe there's some fertilizer root burn too?

All you can do now is wait. Position it in part sun for now and wait for that one little claw to further mature and open up before you can actively resume watering again.

Thank you @tarev! The entire stem is slightly softer than my other plants, but no overly soft spots.

I uprooted it yesterday to check for rot down the stem, or for root rot, and none seems obviously present. I was hesitant to disturb it since planting, but wanted to check for rot. I also mixed in more (plain) perlite to better aerate the soil. It's exposure now has part sun as you've suggested.