This problem indeed exists in the currently available build of World Clock, and is specific to Win 10, because of apparently broken theme drawing API.
Ideally, you want Microsoft to fix their mess, and not release half-done / half-tested product to the market.
You can try to use a few clock objects where each one outputs a single line. Awkward, but may fix the output while a more decent solution, or a fix becomes available.
The problem is that we are chasing a moving target with bugs in it (operating systems coming out of Redmond). Things change, things break, and attempting to have workarounds for everything over-complicates stuff.