There's a bug in Exchange 2019 CU12 and earlier that causes New-MoveRequests for Public Folders to fail.
The move request will fail with the error: StalledDueToMRS_Quarantined, which means that the Mailbox Replication Service (MRS) on the target Exchange 2019 server has crashed repeatedly due to the bug and has quarantined the move request (not the mailbox).
If you check the move request report with the Get-MoveRequestStatistics <PFMailbox> -IncludeReport | FL Report cmdlet you will see the error:
StatusDetail : StalledDueToMRS_Quarantined
Message : Request was quarantined because of following error: Object of type "Microsoft.Exchange.Data.Storage.PublicFolderSession" cannot be converted to object of type "Microsoft.Exchange.Data.Storage.MailboxSession"
Unable to cast object of type 'Microsoft.Exchange.Data.Storage.PublicFolderSession' to type 'Microsoft.Exchange.Data.Storage.MailboxSession'
Microsoft is aware of the problem, which will be fixed in an upcoming Exchange 2019 Cumulative Update. It's unknown at this time if the fix will be included in Exchange Server 2019 CU13.