If an entity (lets take a player as an example) recieves damage on the same tick as it heals, it will not take damage nor knockback (unless its explosion kb). This can be easily replicated by having a friend hit you with a diamond sword while you have regeneration 5 applied and are not wearing any armor (by eating a notch apple): your friend will get the first hit, but wont be able to get any afterwards until you have healed back up to full harts. This is an extreme example of course but is a way of replicating it easily.
The only source of damage which seems to ignore this is the instant damage effect. The cause for this irregularity might be related to MCPE-39732