Different desktop environments deal with app launchers (my way of referring to the *.desktop files) and some apparently put a priority when there is a conflict between the same filenames in different locations.
-distro installed system wide *.desktop files
/usr/share/applications/
-third party specific *.desktop files
/usr/local/share/applications/
-user installed for a specific profile
~/.local/share/applications/
-flatpak *.desktop files installed system-wide
/var/lib/flatpak/exports/share
-flatpak *.desktop files for a single profile
~/.local/share/flatpak/exports/share
So for @ahn both app launcher files (the gramps.desktop file for both the distro installed Gramps 5.1.6 and the flatpak Gramps 5.2.3) are installed in different places, but the desktop environment they have for OpenSUSE places a priority on the distro gramps desktop file while ignoring the flatpak’s desktop file. A further restriction some desktop environments place on flatpak desktop files is that they only check the flatpak desktop directory for changes while loading the user profile.
So the easiest solution here since the flatpak 5.2.3 is correctly installed, if Gramps 5.1.6 is not needed, is to delete the Gramps 5.1.6 provided by the distro and reboot (or log out and log back in). Once the system logs back into the user profile, the desktop environment should put the flatpak Gramps 5.2.3 launcher in the menu now that the distro-provided version of Gramps is removed.
Other workarounds if both 5.1.6 and 5.2.3 are wanted would be to use the flatpak run command in console if the flatpak is wanted, or to rename or modify the Gramps .desktop file in /usr/share/applications so that there won’t be a conflict between the two desktop files in separate directories. I don’t know if a simple rename would be sufficient (like gramps.desktop to gramps-system-install.desktop in /usr/share/applications/ using root or sudo privileges), or if the name entry in the desktop file has to be edited with a sudo nano command or opening the text editor graphically with sudo privileges. Another workaround could be to copy the flatpak’s desktop entry over to /usr/share/applications/ under a name like gramps-flatpak.desktop, but then it would have to be manually removed if the flatpak is removed. Of course all this is irrelevant if the OP just wants to use the latest version and doesn’t care about keeping the older distro provided version.