Thorns tick damage makes the receiver invincible on vanilla multiplayer servers, which is not the case on vanilla LAN worlds. This leads me to believe this could be a networking issue.

This is unlike other instances of indirect damage like fire/poison/wither/freezing/fall damage, where they block knockback but not damage. These sources behave like this on both vanilla multiplayer servers and LAN worlds, thorns tick damage is the only source with this bug, and specifically on vanilla multiplayer servers.

In modern 1.9+ PvP, thorns is massively problematic because of this. If a player with thorns armor attempts to hit a player that just hit them, they will deal no damage because their thorns made the other player invincible. Servers have gotten around this by never giving thorns in kits because this issue affects gameplay so much.

See the attachment "Minecraft 1.19 - Multiplayer (3rd-party Server) 2022-07-26 22-51-01.mp4" for a video of the bug happening on a vanilla multiplayer server._

See the attachment "Thorns Bug 1.mp4" and "Thorns Bug 2.mp4" for videos of the bug happening on a 3rd party multiplayer server.

See the attachment "Thorns Bug 3.mp4", "Thorns Bug 4.mp4", and "Video_2023-11-09_22-54-41-8MB.mp4" for videos of the bug NOT happening on a vanilla LAN world.