What does
Get-Mailbox <user> |FL LitigationHoldEnabled, InPlaceHolds
show?
Since you migrated from on-prem it should show
LitigationHoldEnabled = True
What you are seeing is normal since on-prem uses the classic lit hold architecture and enables the hold across the mailbox.
InPlaceHolds would be populated if you were to place a mailbox on hold using the 365 Compliance/EDiscovery tools