What is it running (per About screen)? I’m also curious how you rolled back the update.
Updates are stored separately from the original install, and might need manual removal.
See Downgrading / reverting to a lower version for some of the potential challenges, but
Release: 2.0.6.1 (beta) 2021-05-03 describes a DB edit that may help in some cases…
Regardless, if the update notifier goes off again, an install from .msi is always possible.
Make sure Duplicati is fully down, and it will probably give you the most reliable “update”.