I had the same problem which I raised in another thread:
At the time I thought I had fixed it by uninstalling the Texlive-fonts-extra package to remove an alternative set of courier fonts.
What is strange is that I re-ran the fc-list | sort | grep -i courier command just now and it shows that I currently have the texlive courier fonts installed again! However, the code is being displayed correctly in the wxPython Demo.
When I did have the problem I was running wxPython 4.1.1 on Linux Mint 20, but I am now running wxPython 4.2.0 on Linux Mint 21.1.