Try changing the video driver in retroarch to vulkan or glcore (working for me). The gl implementation is currently busted but you can try the workaround: https://docs.libretro.com/library/dolphin/
There is also currently a bug with this core and the GL driver that can be worked around by going to settings > user interface > show advanced settings ON and then going to settings > core > allow cores to change the video driver OFF.
The Dolphin core will now work without issues.