My first guesses are...
Have you moved any files within Clavian/SuperTextMesh/Resources? There's some files in there that determine default materials, so maybe it's setting something to null...
Is this happening consistently with another action? Such as... entering playmode, editing an STM object... I'm guessing the letters change to pink squares when this happens?
Are you using HDRP/LWRP code that interacts with STM's shaders? I was able to find this from someone else that looks similar: https://answers.unity.com/questions/1592342/unity-20183-hdrp-change-material-col... Maybe the names inside a script aren't matching up with the variables inside STM?