I've just tried uploading to PM via USB cable from my Canon R5 MkI with the same lack of success as the OP with the MkII. However if I use the cable with Canon Utility, it sees the card & all its files so I guess there's something in Canon's firmware stopping third party programs from reading the files.
Having said that, I wouldn't bother with a cable normally as I agree with Kirk that using a good card reader is the most reliable & expeditious way to go.
-Dave M