If you use System Center Configuration Manager (SCCM) to manage updates deployment, OS deployment and additionally keeping up to date your OS image (WIM), you need to know that SCCM offline servicing is incorrectly detecting Windows Server 2019 WIM image as Windows Server 2016. This has been confirmed as a bug in SCCM and should be fixed with the next SCCM update.
In the interim you can either continue to deploy your Windows Server 2019 and update it after deployment or update your Windows Server 2019 WIM images without using the SCCM offline servicing feature using the DSIM command line
DISM /Mount-Wim /WimFile:C:\Temp\install.wim /index:1 /Mountdir:C:\Temp\Mount – where the index number is the Windows version to update
DISM /image:C:\Temp\Mount /Add-Package /Packagepath:C:\Updates\
DISM /Unmount-Wim /Mountdir:C:\Temp\Mount /commit