Yes, it’s Polish, you got it!. This seems to be happening to some non-Polish characters, as well. I tested a few, and – for example – “á” gets rendered but “ā” doesn’t.
Some characters that don’t get rendered: ą, ę, ń, ł, ż.
Testing it further, I discovered something very weird, though! If they come first in the link, they get rendered. If they are between two other letters, though, they disappear. I also noticed that sometimes they don’t get the capitalized treatment while I type, and typically they disappear after that. It’s all very weird.
Thanks for the help. I really love your theme!