I use SysVinit, KDE (x11), AMD CPU, AMD GPU. The Steam flatpak runs good for me.

I have a feeling the problem might actually be with Gnome or DEs using GTK.
I think that because Gnome (the most) hates x11. I suspect Mint cares little on working with x11 and are focusing on Wayland.
Display servers can affect the init framework and dbus, overall your GPU may not be communicated with correctly.

Options:
1) I haven't ran Cinnamon in a long time, but doesn't it have an option for desktop shortcuts, under Properties, to 'Run with dedicated GPU'? Try running the native Steam app with that (or if you make a desktop shortcut for the Steam flatpak, try that too).

2) Make sure Mint is using the NVIDIA card.  nano ~/.local/share/applications/steam.desktop. Find "Exec=" and modify the line.
Exec=__NV_PRIME_RENDER_OFFLOAD=1 __GLX_VENDOR_LIBRARY_NAME=nvidia /usr/games/steam %U
Then restart computer.

3) Try a Qt based DE.