Not sure what changed but after some core18 or snapcraft/snapd update, themes appear to be broken again, if you build with snapcraft right now. Previously built snaps still work fine.
More info https://github.com/keepassxreboot/keepassxc/issues/2966
EDIT: temp workaround is to specify the default Qt fusion theme by using the style flag, for example:
apps:
symphytum:
command: desktop-launch symphytum -style fusion