First, thank both you for the responses! I would have cried if this post sat empty…
Excellent questions! My PC is a custom built tower. I included some of the details in my signature. All outputs are coming from a single graphics card: an Asus STRIX (GeFore GTX 960)
The manufacturer claims that it can handle 4 concurrent displays, and supports 4K.
There are actually 5 video outputs on the back of the card. One DVI, one HDMI, and 3 DisplayPort.
I am using the DVI and two of the DisplayPort connectors. (One of the DisplayPorts being for the 4K.)
Thinking about it now… it seems more weird to me that it would work than that it would not… 
Still, as far as that goes, I can run just about any other multi-window software across the screens just fine. The desktop acts normal, etc. Every now and then there’s some windows DPI scaling issues on some applications, but that’s about it.
I do not have Wavelab, so far Cubase is the only DAW I rely on. The other software I use a lot is VisualStudio, which handles multiple monitors just fine. I also have the freeware DAW “LMMS”, but I don’t use it much. It has worked so far, but it’s also pretty simple. (I mostly just keep it around because of some old projects I am sentimental towards. They are trapped in it, due to it not being able to export MIDI 
)
Does that help lead to any clues at all? It’s still a mystery to me…