I haven't had this issue with axepad specifically, but I have had similar issues with snap packages.
In my case, I think it was some form of permissions issue that snap didn't like caused by me symlinking the fonts directory from a windows installation on the same disk to the fonts folder in ubuntu (i.e. using sudo ln -s [Windows partition mountpoint]/Windows/Fonts/ /usr/share/fonts/Fonts). Deleting this symlink would rectify such issues for me. In the end, I changed the link to put the fonts in the user's fonts directory and haven't noticed square boxes replacing fonts since.
Based off this experience, have you installed any extra fonts or added symlinks in the fonts directories? It may be worth a try deleting them temporarily if you have.
Otherwise, when starting axepad from the terminal, are there any libraries missing or fonts / permissions like errors? Also checking some of the other logs might be useful, for example /var/log/syslog