The bug
Adult zoglins appear to deal the same damage as a baby zoglin after an adult hoglin is zombified into an adult zoglin. Zoglins spawned by spawn eggs are not affected by this.
How to reproduce
Start world
Run
/summon hoglin ~ ~ ~ {TimeInOverworld:300}Let the adult zoglin attack you
→ ❌ The adult zoglin will almost deal no damage to you similar to a baby zoglin.
I updated the ticket to only include adult hoglins/zoglins dealing less damage. Also caused by the fix of MC-177997.
The cause of this is to do with their attack damage attribute being set to 0.5 (Same damage as a baby hoglin/zoglin) instead of 6.0 damage.