It’s really weird but I’m leaving that information as a solution.
Logging in with GNOME XOrg from login screen fixes that fault with Discord’s black screen problem while screen sharing.
Wayland screen sharing is not nearly as mature as X11 screen sharing.
The simplest and quickest solution is to log out, and before logging in, click on the gear icon in the lower right corner, and select the Xorg/X11 session.
Electron applications can only share the full screen when you are in an X11 session, nonetheless, even in a Wayland session, you should be able to share other X11 applications running in XWayland, just not Wayland-native applications and the full screen.
There is some experimental support for full screen sharing in Firefox and Google Chrome, as far as I’m aware.
See the full topic from here.