Hm, I wonder if it could be...
Please try opening SuperTextMesh.cs, and replace every instance of "t.gameObject.activeInHierarchy" with "isActiveAndEnabled". It could be an inconsistency between active and enabled states, where I really should be requiring both at all times. There's a check for this related to the materials system too, but I'm not sure if it's the cause immediately.
And OK that's great to confirm it's the materials system. When the font or material is different, it's fine... Hm I did do some changes a few months back to code there that determines if two materials can be shared, I wonder if there's a value that's not being considered yet that could be causing the old material to be overridden by the new one, breaking the old text. Do you have a material that's being used on both UI and non-UI text? (This technically should be fine, but I could have messed up some values related to stencils, here:)
Try uncommenting this code that's around line 7475:
//does masking value match
//if(MaterialExists_material.uiStencilDepth != Submesh_stencilDepth)
//{
// continue;
//}
I removed this check for stencil depth since stencil ID *should* be enough to determine the materials are different, but perhaps I over-optimized and it's actually required.
If it's neither of these, I'll keep poking around. I feel it's got to be code *like* the commented code above, some value I should be checking that I'm not.