HDMI output inconsistent

I've got a fresh Orange Pi PC Plus 2 with the relevant Armbian loaded onto a new card. It seems to output just fine to HDMI on my workbench at home, using a relatively new monitor and a direct HDMI cable, but when I try other monitors I get inconsistent results. One is a Quimat 7-inch display that just shows a white backlight when connected to the OPi. However that display has issues with other boards so I'm not entirely worried about it. But the other display is a fairly modern ViewSonic monitor that exhibits a more confusing behavior. The monitor only has DVI/VGA so I'm connecting to it through a passive HDMI-DVI adapter. When I boot the board, the POST screen shows up just fine and displays exactly what I would expect for the board and OS. But after a second when it would normally launch the GUI, it just loses the signal completely. If I re-seat the connection it detects that something has been connected, but then goes back to saying "no signal".  The board appears to be working normally and responds to inputs, but I can't get it to display anything. Also this monitor works perfectly with any other device, and the cable/adapter also function perfectly with other devices, so I don't think it's a hardware issue. The fact that I can see the POST screen on it is the other puzzling part, because if it were an incompatible display or if the adapter were the problem, I'd think that the monitor wouldn't work at all.

Is there something I'm missing here? I don't believe there are any special drivers or configurations needed for this monitor, it's just a bog-standard screen for general computer use, absolutely no frills or anything. Why isn't the OS able to use it after POST? Bonus points if you can give insight into the Quimat 7-inch display too, but that one is quirky enough I might just give up on it.

*Edit* I should also mention that the only thing I've done on this install of Armbian is update, upgrade, and dist-upgrade. Literally no other actions have been performed.

Edited January 25, 20188 yr by Meta
Added info on updates to the OS