Okay, I'll download Unity 2021 tomorrow and see if I can find the issue. I have a feeling I know what it could be, and if it's the case, it's not anything I can fix...
If you want to test this yourself, in SuperTextMesh.cs, there's a single "#if UNITY_2017_1_OR_NEWER" line in there. If you change up the code so that directive is always true, does the bug still happen? This directive is so older versions of Unity wouldn't crash when trying to render multiple materials on a CanvasRenderer, as they weren't supported. So my first guess is that Unity 2021 somehow thinks it's older than 2017 and isn't rendering quads because of that.
Another quick idea, do quads render when you build the project? I've had similar reports in the past where outlines don't render in-editor, but in builds they render just fine. I doubt it's this (and I hope it's not this...) but it's something I'd check anyway.
Just today a few friends were telling me that Unity 2021 is a beta of sorts, so it's possible this could be something out of my control for the time being, just as a heads up! In an absolutely worst-case scenario, I'd write a script to pin non-UI objects to the UI, but hopefully it won't have to come to that.
I tried removing that conditional, but it didn't seem to have any effect. Quads don't seem to render in builds either for my main project (iOS build), or test project (Mac build).
Definitely understand 2021 is hot off the presses and there might not be much you can do at this point. I'd been holding out for point light shadows, but now that they're in, URP perf gains are just too much to pass up. Worst case, I can just rework things to avoid using quads until you're unblocked. Thanks for looking into it!
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
