Okay yep, I'm able to repro it now. (I'm testing this with courier btw. Top mesh here is STM, bottom is UGUI)
I think this is caused by... the middle alignment only takes the width of the letter into account, not the width + advance. When I right-align a UGUI Text component, the text is not flush with the right side of the box, but with STM it is. This is a great catch, but I'm worried about changing this, since it would effect all text meshes, not just monospace fonts. I'll have to come up with a way for this behaviour to only effect monospace fonts automatically, somehow.
I'll write this down for sure, but if you align stuff to the left, it should work for your purposes for now?