When mobs take damage, and seem to ignore knockback when hit by the player(in creative and survival), but as soon as they move away (from what is causing damage to them) they can be knocked back again.

Tested with fire, magma block and cactus.
Does not happen while the mob is in a minecart or when a undead mob is set on fire from sunlight.

When a mob attempts to attack a player that takes damage from those sources, the player isn't knocked back either