I am seeing this at different customers now, that VAMT 3.1 included in Windows 11 ADK 22H2 does no longer accept GVLK product keys for Windows Server 2019 Std. / Datacenter. VAMT does not accept the GVLK keys.
Windows Server 2022 and 2012, 2016 can be added without issues.
The issue was introed with the 22H2 ADK, and does not affect VAMT in 21H1 ADK.
ADK W11 22H2 is still preferable as it fixes DB incompatibility.
Thanks for looking into this.
Install W11 22H2 ADK > VAMT
Open VAMT > go to product keys > try to add WS 2019 GVLK into a blank database.
(if these exist from an older VAMT DB, they will still exist / work).