I’m not a Linux user but an updated Flatpak for Gramps 5.1.4 exists. Is it not compatible with Debian?
[There’s a Debian distro logo in the setup guide like from the Gramps Flatpak page. But that just tells me the Flatpak project is intimately involved with Debian. It doesn’t indicate if the 5.1.4 Gramps Flatpak has been validated with Debian - Buster or bullseye. ]