I'm sure everyone on this thread has already figured out what they needed to figure out, but I thought I would add my situation and solution, in case others had issues relinquishing worksets while synchronizing.
I ran into the issue where it continuously was not relinquishing one of our user-created worksets. Everything was set to "not-editable", and the "Borrowed Elements" checkbox was checked, everything else was greyed out. When I changed the workset to "editable", then opened the sync dialogue box, the "Borrowed Elements" checkbox was no longer checked (because I changed myself to Owner, rather than Borrower), and the "User-created Worksets" checkbox was no longer greyed out, and unchecked. By checking the checkbox and synching, I once again went back into the worksets dialogue, found that specific workset that wasn't relinquishing as relinquished, and changed back to "non-editable". It may not fix the problem for everyone, but it seems to have fixed the problem for me.
I am on Revit 2021, with a BIM360 model, for those wondering.
Hope that helps someone who runs into the same problem.