Right, a few updates ago I remember changing the behaviour for enabling STM while a parent gameobject is inactive...
Are you enabling the STM component, or the entire GameObject it's on in this scenario? Or does it not make a difference between the two? Confirming this will help narrow down the issue greatly - I see some points in the code where I'm only checking for one of these. This and the fact it's happening to all STM objects makes me think it's some type of desync with the internal materials system.
Do all the effected meshes have the same font and material? Or do they have different settings? If it's only happening to a specific font, that would mean it's related the materials system for sure, which automatically makes text meshes with the same settings share internal materials.
The materials system is basically... Upon rebuilding, STM objects will create and use a cache of materials to render, not the actual material assigned in the inspector. I think toying with these created materials could also cause problems, but I'm not sure how they'd be getting edited externally here. Could there be code in your project that's somehow effecting created materials...?
For the animation event you're talking about, does this refer to a Unity animation event, or STM's drawAnims and event tags? Another guess is... if it's not forceAnimation not being enabled, it could be another variable that's being set automatically. Are you only enabling/disabling the STM object? What is being called in the animation event exactly? With autoRead disabled, I believe it would still call Rebuild() and if the mesh has a readDelay above 0, it would idle on frame 1. (I still don't think it's this but more detail is good!)
Also, what version of Unity are you using? I just remembered that Unity 2019 specifically had some issues with rendering multiple materials on a canvas which I was forced to eventually give up on after seeing it worked fine in 2018 and 2020.
Thank you so much for your patience by the way, I understand how it is to have an issue you can't really show off, so I hope this is at least making a bit of progress!