I'm not sure what the issue is exactly but when summoning a gate in 1.21 the mobs that come from the gate will attack you without giving you immunity frames

I'm not seeing this behavior (testing with the Gateway of the Emerald Grove). Zombies from this gateway are unable to attack any faster than normal zombies, and the invulnerability timer is properly raised on the player when the attack hits.

Is there a reproduction case?

Shadows-of-Fire/Apothic-Attributes@f90fabc might be related and may solve this problem, depending on what you were seeing.