Using a user with enough administration right, you can execute the following Powershell on each impersonated mailbox concerned.
$UserCredential = Get-Credential
$Session = New-PSSession -ConfigurationName Microsoft.Exchange -ConnectionUri https://outlook.office365.com/powershell-liveid/ -Credential $UserCredential -Authentication Basic -AllowRedirection
New-MoveRequest -Identity "firstname.lastname@example.org"
You can check the status of the execution of this command by:
Get-MoveRequest -Identity "email@example.com"
You can also use
Get-MoveRequestStatistics to get the progression of the MoveRequest.
The only explanation from Microsoft about this issue is
something has become corrupt and moving the impersonation mailbox
to a different database causes some form of rebuild