I created a Permission Set Group with one Muting Permission Set and was able to retrieve the metadata fine (using source:retrieve -m). However the metadata seems to only contain the name of the Muting Permission Set, not its content. So how do I retrieve the Muting permset itself? I don't see a new type in the MDAPI doc, and couldn't seem to be able to retrieve it as a regular permset (using the API name) either.
Edit: error message trying to retrieve MutingPermissionSet as suggested 