The bug

The recent spawnEvent option in /summon creates corrupt mobs the majority of the time. It causes the regular minecraft:entity_spawned to be skipped, which many mobs rely on for initialization. Mobs that skip this event are permanently broken or corrupted, each in a different way depending on how that mob behavior is implemented.

How to reproduce

1. /summon sheep ~ ~ ~ 0 0 wololo
2. Try to shear the sheep
→ You can't, this mob is missing a bunch of components that were supposed to have been applied by the entity_spawned event. It is now broken permanently.

Notes

Even if the specified event is run after entity_spawned, it would still be possible to create corrupt mobs, for example a chicken that has both adult and child components at the same time. Most mob events are implementation details of the behavior and probably should not be exposed in /summon alone, or a distinction should be made between creation events and lifetime events.