Hey! I might end up implementing that workaround for now, so go ahead and use it. The next focus for STM is optimizing Rebuild() anyway. I think this bug is caused by OnFontTextureRebuilt() not firing, but I'm not 100% sure this time.
This is really good work narrowing down the bug though, I'm not at my computer right now, but I'll give it a proper look soon! If you can send me a sample project that repros the bug, that would help a lot!