I really should be asleep - but I keep thinking about what could be causing this (even though this is a new version of Unity) Maybe... I'm guessing changing the canvas type from screenspace to worldspace doesn't make a difference?
In an extremely desperate scenario, I could write a script that automatically uses custom STM events to spawn new STM objects for characters that require additional materials, I think... Yeah, it should be doable, just not really favourable.
I'll keep thinking about what could be causing this, I'm not finding any issues related to CanvasRenderers and multiple materials on Google just yet, but 2021 iiiis pretty new. Hopefully this will be one of those issues that just solves itself, but I could file a bug report with Unity... I'm just not sure if this is my fault or Unity's just yet