[1.21.8-10] [Forge] Mixin Injector fails on forge

[Fealtous]

Minecraft Version

1.21.10

Mod Loader

Forge

Describe the Issue

Relevant conversation with Llamalad since I didn't understand the specifics of why the injector fails and why it just works:tm: for the loaders using a different mixin flavor.

Tested a few versions in case this goes back several, appears to only be an issue on 1.21.8 and up, which presumably corresponds with the addition of said mixin at this commit 5b6b36e

Logs

https://gnomebot.dev/paste/mclogs/1DpcoqA (End user's log)

or a simplified log with minimal requirements to replicate (personal log):
https://mclo.gs/FJJ0eia