There have been bug reports going back some time.
Typically https://gitlab.gnome.org/GNOME/gimp/-/issues/2826 which references those rectangular characters
Which refers to https://gitlab.gnome.org/GNOME/gimp/-/issues/1366 Gimp crashing at startup due to font issues. (edit: look at the last entry if you are using LibreOffice)
Could be a bad font somewhere.
Typically https://gitlab.gnome.org/GNOME/gimp/-/issues/2826 which references those rectangular characters
Which refers to https://gitlab.gnome.org/GNOME/gimp/-/issues/1366 Gimp crashing at startup due to font issues. (edit: look at the last entry if you are using LibreOffice)
Could be a bad font somewhere.