Some resources I found:
https://www.reddit.com/r/unity/comments/18uwwz9/is_there_a_limit_of_only_like_10...
https://medium.com/my-games-company/how-to-optimize-uis-in-unity-slow-performanc...
In my reproduction, if I give HexContainer a canvas, STM's quad renders again. So I think it might just be a Unity limitation on how canvases with many elements are set up. (Maybe once it reaches 1024 elements, it gives up on rendering submeshes? I know Canvases do *some* type of mesh optimization since UI shaders receive verts in World Space rather than Object Space, and default Unity behaviour tends to not like submeshes as a concept (quads use submeshes to render in STM), so I wouldn't be surprised by this.)
So... I think it might be best to split up the hexes and STM on different Canvases! Seems to be a better way to structure a large scene.