In Windows Server 2019 RDS (Not specific to RDS) users default printer may reset from the default printer they previously set each time the server is rebooted.
You will need to edit the local GPO to stop Windows from managing default printers
On the RDS Server open Local GPO Editor
Windows Key + R
gpedit.msc
Browse to User Configuration > Administrative Templates > Control Panel > Printers
Set the setting ‘Turn off Windows default printer management’ to Enabled.
Perform reboot or gpupdate /force of the RDS Server.
If the above group policy does not work it may be due to the way end users connect via their RDP shortcut, If printer redirection is turned on that default printer on their local machine will be selected rather than the default printer for their RDS user profile.
To fix this apply the below group policy setting on the RDS server.
On the Server go into the Local Group Policy Editor:
Windows Key + R
gpedit.msc
In the Local Group Policy Editor navigate to Computer Configuration > Administrative Templates > Windows Components > Remote Desktop Services > Remote Desktop Session Host > Printer Redirection.
Enable the Do not allow client printer redirection rule to prevent the server to install “Redirected” printers.
Perform a gpudate /force on the RDS Server or perform a reboot of the RDS Server for the GP setting to be applied.