Update Compliance stale devices

lalanc01 ,  there are two things probably still affecting your data.  One is that if the devices still have the commercial Id used for Update Compliance, they will still report in your Update Compliance reports, until that commercial Id is removed.   After that, data is retained for 28 days.

So, the first step is to remove the commercial Id.

Coming soon, Update Compliance will use AAD tenant or device Id's instead, and so removing a device from AAD tenants should remove them from Update Compliance once that takes effect.

HTH