I am unable to reproduce this in a Unity version higher than 2017... (multiple materials are not supported by the canvasrenderer class in Unity versions before 2017) what version of Unity are you using? Would it be possible to get a screenshot of the entire Unity editor, including STM's inspector?
I'm also curious what the inspector of the parent canvas looks like, if additional shader channels are disabled, this would also stop additional fonts from rendering. STM should set this value, but it's possible it could have been changed somehow.
It seems that Unity 2021 has some issues with rendering multiple materials on the same gameObject... it's been a few months into 2021 now, so I should probably try contacting someone at Unity about this. For now, all I can really recommend is sticking to Unity's LTS releases (2018.4, 2019.4) if that's a possibility. Other developer friends have told me that 2021 has a ton of odd things going on currently.
Is this happening with just URP/HDRP, or is this with the standard 3D system? This will help me file the report to Unity.
Edit: Checked with other asset devs too, and it appears that Unity 2021 is a "tech release stream" so it's still in beta and a lot of stuff doesn't work. If possible, try one of the LTS releases!
it is your example scene "InlineFontChanges". i did just move world to UGUI Canvas.