It ended up being an issue with certain fonts not being installed correctly. Unfortunately, that's all the detail I can offer; I resolved this months ago and had forgotten about making this post at the time of the fix.

I managed to fix my other Fedora 31 machines. You also have to remove the packages texlive-sourcecodepro, texlive-sourcesanspro, and texlive-sourceserifpro. It might be that texlive-sourcecodepro is enough, but even so, a lot of extra texlive packages are removed along the way because of dependencies on those three font packages. None of these other packages look critical. However, a lot of Mathematica users also use TeX, so there may be a more serious problem lurking here.

If you're still seeing that weirdness on Ubuntu, I'd suggest looking for installed fonts that overlap with the fonts in /usr/local/Wolfram/Mathematica/12.0/SystemFiles/Fonts/. Maybe removing one of them will help, but it might make some other program unusable.

Saving as pdf works perfectly for me (using Kubuntu 18.04):

In[3]:= $Version

Out[3]= "12.0.0 for Linux x86 (64-bit) (April 7, 2019)"