I can confirm the same bug with the now current 6.1.5-3+rpi1+deb10u5(ruinning on a raspberry PI 4).
Removing libreoffice-gtk3 made writer start without issues (but without theme integration). Writer displays no issues with libreoffice-gtk2 (and runs with the correct theme). If I reinstall libreoffice-gtk3, the issuereturns.
I can attach strace and package info if necessary, but it looks relatedto gtk3 theme integration. Thank you!
-jan