What type of neem oil are you using?
Cold pressed 100% neem oil is the only way to go. Mix with a drop of dish soap or another wetting agent and mist with a spray bottle.
DO NOT MIST THE INFLO. And only spray the plumerias right before the lights turn off on your grow lights.
If you spray it on while the grow lights continue to shine on them you'll give the leaves sun burns and yes they will turn yellowish and fall off.
I had my plumerias under grow lighst this winter and had 3 inflos that did not produce. Lots of branching like you but no flowers.
I have come to the conclusion that neem oil was the cause of two which I drenched them with the leaves. (I had a bad infestation) The other one I'm not sure about. No humidity in the studio under grow lights so that could be the cause too.
