Huhhh is this due to custom code editing the materials, or just as behaviour for STM? Just checking to be sure.
I made an update to UpdateMaskingOnAllSubmeshes last month, so was wondering if something like this would happen... still not sure why sampleInfoIndex would get out of sync with info.Count (use infoCount by the way, *that* value should be synced) but yeah I think making sure Submesh_infoIndex is greater than -1 and less than infoCount and returning otherwise would be fine. I'd really just expect this to happen when loading or something like that and correct itself, but if you can figure out the exact timing it happens, please let me know!