When summoning different variants of dogs(wolves) and cats, the alphabet "v" of NBT "variant" in command，MUST lowercase "v",otherwise capital "V" ONLY spawns default variety.
For example 
/summon minecraft:wolf ~ ~ {variant:black,NoAI:1,NoGravity:1,Silent:0,PersistenceRequired:1,Invulnerable:1,Owner:bruh}
spawns black wolf.
Otherwise
/summon minecraft:wolf ~ ~ ~ {Variant:black,NoAI:1,NoGravity:1,Silent:0,PersistenceRequired:1,Invulnerable:1,Owner:bruh}
STILL spawns default white wolf.
However parrots and other mobs not found this bug.

The version 1.21 has already had the bug.