Hi,
I asked in February 2021 whether this was still reproducible on a current
Bullseye or Sid install, and never got an answer.
For the record, in your screenshot the GTK window of reportbug renders all
its text correctly, while only the text drawn by Cinnamon itself shows as
white boxes. So this was not a missing font, but the glyph rendering path
of the old external Clutter/Cogl 1.x libraries, which depends on the
graphics driver in use, and the report contains no information about the
GPU or the driver.
Those libraries are gone anyway: since cinnamon 5.4.12-2 the package no
longer uses gir1.2-clutter-1.0 and gir1.2-cogl-1.0, and the panel text is
rendered through the maintained clutter/cogl fork bundled in muffin, taken
from a modern mutter.
I'm therefore closing this bug. If you still see white boxes in the panel
on trixie or sid, please open a new report including your GPU and the
driver in use.
Thanks for the report,
Fabio