I’m sorry if it’s still not clear. This bug also happens with the VANILLA Official server provided by Mojang. The video I recorded showing the bug was recorded on the official server by mojang.
I'm sorry, I forgot to mention that. This bug also happens with the OFFICIAL Minecraft Server implementation, downloadable on the official minecraft website.
The video was recorded using the official vanilla minecraft server, as well as a vanilla client.
The problem is present with mobs that attacks in close combat. Projectile like arrows do damage to the shield, as they should.
The bug is present in all versions from when the shield was added (1.9) to the latest (1.21.8).