The bug
When a flower is on a edge, bees have a hard time getting to the flower. It seems like bees have trouble pathfinding over block edges in general.
To reproduce
Summon some bees
Place some flowers on an edge, with no flowers being anywhere else
→ ❌ The bees won't be able to pollinate the flowers
This problem is the worst in flower forests, a biome with "random" terrain generation, as there are many slopes, edges, and floating terrain in this biome. This can be reproduced in a flower forest buffet world as many bees are outside their nests trying to get pollinated but they often can't.