I'm currently running Windows 7 Home Premium. (I got an absolute dinosaur ik.) And after I stumbled on the other comment (the one with the photo) I noticed they said they used Windows 8. Maybe it's an issue with old OS's? If it helps at all, transition images seem to be affected too, so it's not purely text. (Which tosses out my theory that it's italicized a million times over, parts of the images are being directly manipulated.) Shoot back with any information I can help you guys get, I'm getting more and more curious about this!
Heya Talo, so it's been a bit but if you haven't seen our latest DevLog, we believe we've isolated the issue with the new Ren'py engine. (It helps that there was a log that explained the issue.)
https://www.renpy.org/doc/html/display_problems.html
Give the instructions here a try, and see if you're still having issues.