A while ago, there was a glitch where if you got a mob down to exactly 3 hearts, then used dispensers to splash them with a harming and healing potion simultaneously, they would glitch out and become invincible. This was patched about 2 years ago, in 1.19.20.

However, upon updating to the latest preview, all mobs obtained with that glitch will simply vanish. Lots of people really enjoy having these mobs, and it would be a slap in the face to those people.

How to reproduce:
Do the old glitch to make a mob invincible (you can probably find tutorials on youtube)
Update to 1.21.70.22

Expected results:
The mobs remain in the update, and stay invincible.

Actual Results:
The mobs simply vanish