I had this issue when I was using -F fsr, you could try disabling that if you're using it. I also have to do ENABLE_HDR_WSI=0, since I have that enabled for MPV but setting it to 1 makes Gamescope break.
My current launch options are ENABLE_HDR_WSI=0 DXVK_HDR=1 gamescope -f -H 2160 --force-grab-cursor --hdr-enabled --hdr-debug-force-output --hdr-itm-enable -- env MANGOHUD=1 %command%. I may also have ENABLE_GAMESCOPE_WSI=1 set as well, you could try that.
Using NixOS with Linux 6.8, Plasma 6.1, gamescope-wsi_git, and an RX 7600.