did you try running the snappy-debug tool (from the snappy-debug snap) in a second terminal while trying to use your snap ? that should show if there are any interfaces missing (i doubt the x11 plug alone is enough to get access to all desktop ressources)
Mutter in GNOME 3.38 was changed to not listen on the abstract namespace X11 socket when running a Wayland session. Snapd’s x11 support relied on access to this socket. This change was reverted in Mutter 3.38.2, but your distro may not have picked up that update yet.
Things work in Ubuntu because releases before 20.10 ship older releases of GNOME, and for 20.10 we back ported the mutter fix.
On the snapd side, we’ve made changes to make the non-abstract socket available to snaps, which should make them functional on systems where the abstract socket is not available. That change landed in snapd 2.48 though, which is newer than what you appear to be running.
So the solution comes down to doing one of the following: upgrade mutter, upgrade snapd, or switch back to the X11 session.