I have discovered a compatibility issue with the Windows Deduplication feature on Windows Server 2019 after the 2021-06 Cumulative Update (KB5003646) is applied.
If the Windows Server 2019 machine has a volume attached which was originally deduped on a Windows Server 2016 machine then it will BSOD relating to dedup.sys.
The dedup.sys driver for Windows Server 2019 was updated between 2021-05 CU (file version 10.0.17763.1554) and 2021-06 CU (file version 10.0.17763.1971).
I have reproduced this issue with a clean install of Windows Server 2019 from the RTM ISO with no additional software or configuration changes. Up to and including OS build 17763.1935 (2021-05 CU) there are no issues accessing data on a volume originally deduped on Windows Server 2016. Once the system is updated to OS builds 17763.1999 (2021-06 CU) then accessing data on that same volume will cause a BSOD relating to dedup.sys.
The issue is not present for volumes which were first deduped on Windows Server 2019 only when the volume was first deduped on Windows Server 2016.
This was originally discovered when performing a rolling OS upgrade of a failover file server cluster running Windows Server 2016 nodes which use deduped volumes for file shares. Once the roles with the deduped volumes were moved to the first Windows Server 2019 node it caused that node to BSOD.