On Windows Server 2019 we have exactly this issue: https://social.technet.microsoft.com/Forums/en-US/c95e831d-36ae-4307-b3fd-a9daac6b5790/full-screen-on-dual-monitor-does-not-restore-correctly?forum=win10itprovirt

Solution in this thread was mentioned to change the WDDM settings in gpedit. This does not fix it in our case. Neither on Windows 10 and not on Windows 8.1 guests as there is no such setting in gpedit in Windows 8.1.
We are using vmconnect.exe to connect toguest with 2 monitors. Both monitors are enabled in enhanced session. Minimizing leads to the issue that monitors are only restored on one monitor not on both. Additionally restoration has errors as scrollbars are being displayed and clicking on icons does not work. . It also poses serious issues with some applications as they cannot easily dragged back to 2nd monitor after restore.
Only solution atm is not to minimise, which is rididulous. Every time we have to exit vmconnect. An re-login. Only this preservers the correct 2 monitor layout state.