Overridden Vanilla Mobs Cannot Be Spawned Correctly By MythicMobs
Steps to reproduce
CAVE_SPIDER: Display: '&7[&fLVL 1&7] &cBaby Hound' Health: 999 Damage: 0 Options: AlwaysShowName: true
Here's an example Mob you can test this bug with.
Current behavior
MythicMobs is unable to spawn overridden Vanilla Mobs using the /mm mobs spawn NAME command. Override a Vanilla Mythic Mob, then spawn one in using the MythicMobs Command. It'll just spawn a default Vanilla Mob, instead of the one overridden by MythicMobs. Although if you use the Summon Command or CMI Plugin's Summon Command, and summon in the Vanilla Mob, it'll actually spawn in the correct MythicMob!
Also, these mobs cannot be used with MythicMob Spawners.
Finally...I went back to v4.9.0 and you can spawn the correct Mob with the /mm mobs spawn command. It likely won't help much, but at least you now have a range of versions in which this originated in.
Intended correct behavior
It should spawn the overridden mob...not a vanilla one.