Entities that have a knockbackResistance of 1.0 still get knocked back when shot by a bow with Punch enchantment. I also tested this with the Knockback enchantment on a sword, but in that case it worked as expected.

Expected result:
Entities that have knockbackResistance of 1.0 (100% chance of resistance) should not get knocked back by a bow with Punch enchantment.

Actual result:
Entities with knockbackResistance of 1.0 get knocked back by a bow with Punch enchantment.

How to reproduce:
1. Get a bow with Punch enchantment.
2. Summon in an entity with knockbackResistance of 1.0.

/summon minecraft:creeper ~ ~ ~ {Attributes:[{Name:generic.knockbackResistance,Base:1.0d}]}

3. Shoot the entity with the bow and you'll see it getting knocked back.

I've tested this with zombies, skeletons, creepers and blazes. Don't have time to test the other entities at the moment of writing this, but I'm expecting the same bugged result.

I couldn't find this bug being reported anywhere. I searched for well over 20 minutes.
The closest case I found was related to Knockback enchantment bypassing knockbackResistance, MC-45077, but that got fixed.