Hi Darren,
Then this is really strange. The Plastique style doesn't use the system theme engine, so it should work in a predictable and bug-free way on all platforms. For some reason that I can't figure out, even Plastique is not rendering fonts correctly on your machine.
Which version of Windows are you using? Have you customized your screen preferences (in Windows, not in PixInsight) to set a nonstandard font face and/or size?