I experience the same bug every time I try to open a box dialog to attach or save as something.
This bug should be a problem regarding snap (it happens for other snaps), but I did not try with the .deb version.
A workaround (on Ubuntu 20.04) is the following:
rm -rf ~/snap/mailspring/common/.cache/fontconfig/
fc-cache -rv
On Fedora 34, I have a ~/snap/mailspring/common/.cache/fontconfig/fc3b2af7b38f296247b7fcc731836117-le64.cache-6 but no fc-cache in that folder. Deleting it does not solve the problem.
Do you have to reinstall mailspring afterwards, or just quit and reopen?
On 1.9.2 Archlinux I didn’t have an fc-cache in the fontconfig folder. Tried deleting the fontconfig folder, but it is simply recreated and subsequently see the same issue.