The bug

Pandas can interrupt their eating animation, dropping the item they were eating. If it is bamboo or cake, they will pick it up again.

This could be intentional, as it has been the case since the introduction of pandas (see this comment and the description of MC-206684 acknowledging that pandas drop items they are supposed to eat).

To reproduce

You can reproduce this issue by giving the panda bamboo or cake, but it is easier to reproduce this issue with an item the panda does not eat (anything except bamboo and cake).

1. Summon a panda holding an item. For example:

/summon panda ~ ~ ~ {HandItems:[{id:"minecraft:chorus_fruit",Count:1b}]}

2. Wait for it to start eating.
3. The panda stops eating and drops the item.

The panda almost always immediately picks up the item again if it is bamboo or cake. This can be verified by the "Item plops" sound, and analyzing the panda's hand item via the command

/data get entity @e[type=panda,limit=1,sort=nearest] HandItems[0]

Expected result

Pandas shouldn't interrupt their eating animation and drop the item they were eating.