Steps to reproduce

1. Place a sticky piston pointe upwards one block above the ground
2. place a block of slime on top of the sticky piston
3. place another block of slime next to the other
4. place either a cauldron or a composter on top of that other slime block
5. place a redstone block underneath the second slime block so piston starts moving up and down
6. Now enter in the composter or the cauldron.

Observed results
Either it will glitch and you will be teleported down for a moment and you will be back in the cauldron, or you will be placed under it, or you will be ejected (it seems that you are ejected when you touch the corner in the cauldron/compost)

Expected results
That there is only one behavior, because the fact that it seems random makes it a bug.