This has been around for a while, but I haven't seen anyone report this issue, as many non-vanilla server software have hotfixes for this.

Bug Description

When a held-up shield is broken, a damage-tick-like behavior occurs: the player no longer receives knockback, and only takes damage that is greater than the initial damage within the damage tick. However, the player does not turn red, nor has received any damage on the shield break. See also the attached videos.

Steps to Reproduce

1. Break a player's held-up shield
2. And then hit the player again after roughly 0.4 seconds
3. The bug is reproduced

OR

1. Let 2 mobs (wielding axes) attack a player while their shield is held up
2. If the 2 mobs attack is less than 0.5 seconds apart as well as not at the same time, the bug is reproduced

Expected Behavior

Damage tick does not get triggered when the shield is broken, but on the following attack instead, so that knockback could be applied after hitting twice rapidly, and the player would turn red on the second hit.