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.