The same thing is happening to my plumeria, I've lost three leaves this last weekend. They turned yellow and then fell off. Where I live its been between 100-106 degrees which is a lot hotter than normal. I'm worried the heat might be the cause of mine falling off. Hopefully someone has good feedback on this!

I took these photos this morning and the leaves are even more yellow than they were earlier. A few leaves seem to be ok though. i was expecting the leaves to fall as the weather got colder, just not this soon. :(

Is this what usually happens when it starts to get colder and the season changes? This is my first time growing a plumeria so I'm not sure about all the changes it goes through. Also does it usually flower only once a year?

It's my first time too - best I understand is that as the plumeria goes dormant, the leaves fall off to conserve energy in the cooler months. My big one only had one flower and I was NOT expecting it to flower at all this year so I got lucky.

The branch tips look healthy and I do not see any clear signs of insect issues so I think its just part of the natural cycle.

Generally speaking a pre-dormancy characteristic is the oldest leaves start to get that yellow look which progresses until it falls off or you get tired of looking at them and gently pull them off (there is a right way and a not so right way to pull leaves off). Dormancy or chronic dry conditions do look much the same. Here in South Texas we actually call it the end of summer leaf drop. It typically coincides with the ending of hot summer (July - August) as we transition into a temperate climate with more rain. Historically its also about the timeframe where my rainwater harvested supplies have run dry and am having to water 5+ times per week.

Once a leaf gets old and no longer offers any usefulness to the plant it will eventually fall off. This is from a few days ago under my Duke Plumeria. No harm, no issues, just the time of year and drying out from high wind and being laid over during Harvey.

a Bud G.'s My Valentine showing end of summer leaf drop.

Various 1 and 2 gallon rooted cuttings showing chronically dry.

Brittany,
Each cultivar is different in flowering. Each branch tip has the capability to produce one inflo with multiple flowers each growing season. Each inflo can continuously produce flowers for months or may just go for a couple of weeks and then dry out and fall off. The My Valentine above has been blooming since late May. Long enough that the right side of the inflo is starting to get covered by new growth leaves.

Some cultivars will produce multiple inflos per tip in a season. . Generally speaking younger plants do not necessarily have the capacity to produce a large amount of blooms just yet. In my climate usually by year 3 I am seeing an improved bloom display. Some rather pesky cultivars have taken into year 4 or 5 before showing anything.