Phone model: Redmi 4A
Usually, after taming a donkey, aiming at the donkey with a chest will show an "attach chest" button at the bottom of the screen, which will allow a chest to be equipped to the donkey.
For some of the donkeys that I found in survival mode, after taming them then aiming at them with a chest, the button that appears at the bottom shows "ride" instead of the expected "attach chest", and when I perform the action, my character rides the donkey. This cause the donkey to be unable to be equipped with a chest as regardless of whether I am holding a chest, the button always reads "ride". I tried various things including feeding the donkey, sneaking, and restarting the game, which all failed to enable me to equip a chest onto the glitched donkeys.

EDIT: I have managed to replicate the bug
In creative mode, I spawned a donkey and grabbed a stack of chest from the creative menu, then switched to survival. After taming the donkey, the "attach chest" action works normally. However, I then placed one of the chests onto the ground, then destroyed the chest and picked it back up. after doing so, I can no longer attach the chest to the donkey. In order for the "attach chest" button to appear I had to ditch that stack of chest, then grab a new stack of chest from the creative menu. The attach chest button appears again after switching to a new stack of chest.
So the steps that replicated this bug are:
1. find/spawn an adult donkey
2. get some chests (I had a stack of 64, taken from creative)
3. place one of the chest down onto the ground
4. break the placed chest and pick it up
5. try to aim at the donkey while holding the chests, "ride" button appears and "attach chest" does not