If you try to damage a mob with a sword while it is taking slight damage, it won't knock it back despite taking damage from your sword. This issue is what is being reported in MC-98142, however, the resolution is Intended by a mod because "Mobs have a cooldown period between taking damage. You can't hit mobs if they just took fire damage." This is inaccurate though, because the i-frame only applies if the damage taken after the original damage is less than or equal to that of the original damage. You can prove this by spawning a skeleton, letting it burn in sunlight. It should take 20 1/2 hearts of damage. Then hit a new skeleton during one of its i-frames with literally any tool with >1 attack damage. Note it doesn't take knockback, but you actually did damage it. So the WAI resolution on MC-98142 isn't true.

Steps to reproduce

1. Spawn a skeleton in open sunlight and notice it takes damage 20 times, meaning sunlight deals 1 damage each time (don't use zombies cause they have natural armor so they will absorb slightly more damage)
2. Get any tool that deals >1 damage
3. Spawn a new skeleton and hit it a few times with that tool during its i-frames (while it is still red). Notice it doesn't take knockback.
4. Count the amounts of times it takes sun damage. Depending on the attack damage of the tool, it should be considerably less than 20.
5. This means that the explaination for why MC-98142 is intended is false. Since the mob is taking damage from you, there is no reason why it should be immune to knockback.