VirtualBox second monitor disabled on boot

I just updated VB to 6.1.16 r140961 (Qt5.6.2) and the second monitor gets disabled as soon the OS boots. So I must to extend the desktop to the second monitor every time I start the virtual machine.
Once is working it works ok until I shut off and restart the VM again.
That’s was not happening on the previous release V5.x.x
The GPU is an RX580 With the last Adrenalin drivers.
The host is W7 x64 Home Premium and the VM is an XP SP1