ActivityPub federation is very complicated, and I don’t have a full understanding of it myself. I’ve never come across a situation like this one before, where a user on one instance appears to not exist at all from the viewpoint of another instance that’s federated with it.
I do know that, compared to Lemmy, Piefed is buggy, and PyLova, as a soft fork of Piefed, presumably is buggy as well. You might try asking in a Piefed/PyLova support community. Maybe they’ve encountered this before.