Windows Defender Application Guard (WDAG) for legacy Edge not accepting keyboard input anywhere.

My WDAG for legacy Edge sometimes do not accept keyboard input anywhere. I click on address bar, the cursor starts to blink there and when I type nothing happens. The same happens to the search box of the default page (in my case, MSN).

To make matters worse, I may have WDAG running fine for a moment and, just after a reboot, it comes back to this strange behaviour and no keyboard input again. Since I have made absolutely no change on setting on my own, I really can not understand what is going on.

At first I thought there was a hidden interaction with Controlled Folder Access (CFA) enabled but I do not think anymore that this is the reason. Nevertheless, I fix the problem by turning off CFA and rebooting. As soon as I confirm that WDAG is accepting keyboard input I re-enable CFA. Reboot and it is just fine. I leave the computer on for several hours and no problem, Then, a new reboot and voilà, the problem is back again.

Some hints and info:

* Windows 10, 1909 Pro - not a Windows Insider release.

* Windows Sandbox is fine, but I can not type anything in the search box - in this case it seems to be disabled because the cursor does not appear there. Nonetheless, the WDAG problem aforementioned already exists long before installing the Sandbox.

* CFA is turned on and there is no CFA-related event (according to Microsoft Docs) registered in either CFA history or Windows Event Viewer.

* Languages/Keyboard set:
    en-UK/US international
    en-UK/Brazilian Portuguese ABNT-2
    pt-BR/US-international

* Windows Display Language: en-UK

* Country Region: en-UK

* Reliability Monitor shows only usual software update. No crash.

* DISM/SFC: fine! No problem here.

* I am the administrator of the machine.

* Manufacturer drivers up to date.

This computer has been just reset from OEM Windows 10 1709 and upgraded straight to 1909. WDAG was installed just after that so I could say that there is no old programme causing trouble.