Bougainvillea flowers drying out.
Plant in soil doing well then this happened.
Certified GKH Gardening Expert
Yep! Those are "flowers", and you have a yellow form.
The flowers are actually modified leaf bracts, and some are yellow. I see reproductive organs, so I can safely say that those are flowers.
This article will help you to care for these plants: https://www.gardeningknowhow.com/ornamental/vines/bougainvillea/bougainvillea-garden-care.htm