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.
Commands and nbt/components are case sensitive, this is intentional behavior per the resolution of MC-99199, and the following release article
On a more general note, over the previous snapshot cycles recently data has been far more generally moving towards lowercase syntax in preference to uppercase letters. It is more unexpected that other mobs would work with uppercase letters comparatively.