My moonflower blooms in the morning also. I'm not sure what time they open up but are open when I go out around 8AM. Mine is the bush and not the vine.

Hello ive grown the moonflower vine for two years and it happend but not too often. it happened usually when it starded to get cooler and some of the blooms stayed open from the night before when it got cooler. but it could be a bloom that decided to open later than the rest. are the leaves large and heart-shaped? mine usually start to open at around 5:00 pm like clock work.with luminous white flowers. and it depends on weather you have the bush or the vine. hope this helps