Closely relates to MC-270532.

The bug

If the player attempts to ride a hostile mob, they will eventually die as a result of continuously being attacked by it. I'd have expected that mobs would be immune to the attacks of their riders and passengers.

How to reproduce

1. Find a slime larger than the minimum size

2. /ride @s mount [UUID of slime]

3. Enter Survival

Expected results

You should ride the slime harmlessly.

Actual results

Rapid death.