When you hit a mob during the moment that it is taking damage from fire, it will not react to the hit. It is possible it is not taking damage at all or is just not being knocked back.

Example included as video:
https://youtu.be/7Pa5z0nDcoY

Not seen: also affects passive mobs on fire and mobs on fire from non-weapon sources when hit by weapon.

Only tested on 1.15.2 and release 1.16 release candidate 1, have not tested on previous versions.