I can confirm that on Guix System mpv also fails after the upgrade to 0.35.

Mpv won't play videos but it can play audio in the terminal. When trying to play videos it will give the following error:

"[vo/gpu] VT_GETMODE failed: Inappropriate ioctl for device
[vo/gpu/opengl] Failed to set up VT switcher. Terminal switching will be unavailable.
amdgpu_device_initialize: amdgpu_query_info(ACCEL_WORKING) failed (-13)
amdgpu: amdgpu_device_initialize failed.
[vo/gpu] Failed to commit ModeSetting atomic request (-13)
[vo/gpu/opengl] Failed to set CRTC for connector 66: Permission denied
Error opening/initializing the selected video_out (--vo) device.
Video: no video

Exiting... (Errors when loading file)"

This also sometimes causes the desktop environment to crash and go to the login screen.

A quick google search indicates that this seems to happen because mpv now needs libxpresent to work with x11 (just like Jorge P. de Morais said above):

I would submit a patch to fix this if I was better at guile. Is someone out there up to the task?

Cheers

Markus