Ah got it, since users are uploading their own textures, that makes sense.
I believe that within the switch statement, you're missing...
ParseText_info.submeshChange = true;
...Which signals to SuperTextMesh to switch to a different material when using a quad, which is why it appears to be using the UVs of your quad, but with the font texture instead.
And later, after the switch statement, add the "em" tag to the switch statement that says "ignore single-use tags". (Quads should already be in that switch, so copy the formatting)
Hopefully this works! Thank you for your patience with me, it's been a while since I've modified the code for quads. I'd really love to switch this system to be more modular so you didn't have to edit a switch statement to add a custom tag, someday...