Bees have a tendency to vanish at random, perhaps due to several reasons:

• bees refuse to enter their hives (MC-160027) because they are too full, and will not look for a nearby empty hive
• bees occasionally take suffocation damage from flying into the bottom of their hives
• bees make no efforts to avoid campfire damage
• bees will occasionally wander too far and freeze, until the player intervenes with a flower, lead, or by hitting them, perhaps due to a lack of path-finding after a certain time (as stated in several other errors)
• The tick timer that defines when a mob can breed again will not tick down while a bee is working inside a hive, meaning sometimes bees cannot breed for days on end if they're productive, and baby bees can takes weeks to fully grow up

On top of this, Bees also struggle to get over fences and fence gates, usually ending in them trying to run into them until interacted with/ their path-finding refuses to find another way around obstacles, and treats fences as if they were only a block tall. This is inconsistent however, and they can occasionally climb over fences/walls.

After some observation, bees which have not entered a hive/changed their current path (i.e, from wandering to pollinating) will not flap their wings after several days/ after relogging. the current exact causes are difficult to observe