First, I would suggest you check the join status of the said devices.
- Open a command prompt as an administrator
- Then type
In the result, check the values for DomainJoined and AzureAdJoined.
If DomainJoined field is YES, it indicates the device is joined to an on-premises Active Directory.
Now, to remove the devices completely (then have them rejoin) - Disable or delete Windows 10 devices in your on-premises AD, and let Azure AD Connect synchronize the changed device status to Azure AD.
NOTE: Deleting devices in your on-premises AD or Azure AD does not remove registration on the client. It will only prevent access to resources using device as an identity
Now, to remove the registration from the client completely, make sure to turn off automatic registration. Then the scheduled task doesn't register the device again. Next, open a command prompt as an administrator and enter
dsregcmd.exe /debug /leave
Now, reboot the device. Make sure that the entries have been removed from your On-Premises and Azure AD.
After that run through the hybrid registration steps again.