[MC-140067] Illager beasts drown

See summary. Given that illager beasts are equivalents to iron golems, which are mobs that don't drown, it doesn't make sense, unless there is proof that beasts aren't artificial mobs like the golem.

How to reproduce:

Make a 3 x 3 x 3 hole
Cover it with a ceiling
Fill it with water
Spawn an illager beast

→ It drowns after a while, eventually dying after a long time due to its massive HP.