If two bees are trying to enter a bee hive/nest already ocupied by two other bees, one of them will enter, preventing the second one from doing so. The bee that is left outside will look for another bee hive to ocupy, but sometimes, even when there are other hives nearby, it will fail to do so. In this case the bee will proceed to leave the area moving in the north-west direction (This might not be totaly acurate, but it was the direction they went in all my tests).

This may be the reason other players have reported their bees leaving/disapearing like in MC-166825 .

The video below show this bug in action. There are two bee nests one empty, and the other one with two bees. Once another two bees are summoned, they try to enter the closest nest. One enters and the second one instead of going to the other nest, wanders off to nowere.
In the second the video the bee decides to polinate a flower instead of entering the nest (despite it beeing night) and after he is done he does not enter the nest (maybe flew too far away).