The bug:
Mobs immune to freezing will be killed by the freeze damage of the /damage command; this is unlike mobs immune to fall damage which take no damage with the fall type inserted in /damage.
Using the /damage with drowning type should not damage freeze-immune mobs.
Affected freeze-immune mobs:
Skeleton
Stray
Polar Bear
Snow Golem
Ender dragon
Wither
Steps to reproduce:
Summon any of the above-listed mobs.
Use the /damage command:
/damage @e[type=!player,sort=nearest,limit=1] 30 minecraft:freeze at ~ ~ ~
Result:
The mob dies.
Expected result:
No damage is dealt, and the command returns "The entity is immune to this damage type"
Still in 1.21.4.