There seems to be a bug in PestFeastBehavior where, whenever the the afterReached method is called, the itemhandler is always null, so the pests cannot actually steal the items. Seems like something about capabilities changed between 1.20 and 1.21, and this behavior probably wasn't tested in game.

To replicate: place a chest with food, and spam open it to spawn a pest, note that this will not work in areas colder than -12 degrees because of the new climate restrictions on pests.