I have the same situation with regular piOS and rpi4b. Sometimes the hifiberry-digi is detected and sometimes not with the error "wm8804: probe of 1-003b failed with error -121".
My workaround was:
- remove the dtoverlay=hifiberry-digi from config.txt
- add dtoverlay hifiberry-digi to rc.local (and make it executable if it wasn't yet)