I'm going to bump this because I also have this issue. I have to switch to Xorg if I need to use Steam Link over Steam's normal remote streaming because a game is acting up, but I'd really rather not.

Does this happen when using the Steam Link app on a scaled Wayland display, or when the host is running Wayland with scaling enabled?

I am having this issue too on Ubuntu 23.10 on a scaled Wayland display: 2x scale shows only the upper left quarter of the steam link window. If I drop the screen scaling down to 100% it runs perfectly. If I run the flatpak with the --windowed command, it also runs fine at 2x scaling.

To answer @slouken's question - this happens when the client has a hidpi display with scaling, not the host.

For me on Fedora 39 KDE at 125% scaling it fills the screen correctly, but it sets the host resolution to 1080/1.25=864p, thus the image is ver mushy. Would be cool if the Steam Link app would have an option to ignore window scaling, or at least to set the host resolution to the native res of the client and not the effective res after scaling.