Hmm... looking back at what's changed, could it maybe be the autoRead setting of the mesh? I fixed an error where setting .text would ignore the mesh's autoread settings. But it looks like you're forcing it to read with that Rebuild() anyway, so I don't think it's this... but try setting ._text in that 2nd row anyway, to see if it makes a difference! It's possible that the readTime value is being overwritten...?
I've been meaning to add a backspacing effect myself for years now, but I think one miiight be possible with the new UndoContinue() tag and inverting STM's drawOrder? I can look into this more in a bit! I'm going through my docs and git history to find changes with STMTextInfo and latestNumber.