Thank you for writing.
I have tested the described scenario and found what is causing this behavior. By default we use the GDI+ to measure the text, however, GDI+ is a known suspect for reporting wrong sizes of the text and this is also the case here.
Luckily, you can easily force the element to use GDI instead of GDI+ by setting a single property and this should improve the rendering in this case:
Let me know how this works for you.