Yeah, I think I want a repro project before I work further on this, sorry! I can't seem to repro it myself, even though you're able to repro it 100% of the time. It's gotta be the exact combo of text/font settings you have, maybe...? Causing a texture rebuild at an inopportune time...?
Either way, that bonus call shouldn't be an issue for most situations in-game... especially for STM objects with small amounts of text. The Rebuild() optimizations I'm working on will mainly affect longer strings of text in-editor.