Alright... just got an email so here's what it said...
"""
Thanks for getting in touch, we actually know about this issue and you can check the status here:
https://issuetracker.unity3d.com/issues/the-first-submesh-texture-is-overridden-...
he case is resolved and should be fixed in Unity 2022.1.0a3 and the versions above. We are currently also evaluating requests for backporting the fix to 2021.2, 2021.1 streams.
"""
*Looks at the calendar* well. Hopefully that "backporting" happens. I didn't even know Unity had a 2022 version, the 2021 build is still clearly in beta... I'm going to keep emailing them, so I'll post more updates as it happens. Glad to know this issue finally isn't my fault, though!