Only one Display with 6.1.2?

[ngong-49]

Debian Stretch host and guest (LMDE3), Cinnamon desktop.
I have upgraded vom 6.0.something to 6.1.2, including Extension Pack and Guest Additions.
I used to use 2 displays for the guest as selected in the Settings of guest VM.
Works always as expected, lately with 6.0.

With 6.1.2 it seems VBox does not recognize the settings regarding number of displays.

Is this specific to my installation or is it a bug?
If it is a bug, is there a workaround?
If it is just my machine, how to investigate the subject?