There are so many variables here it’s hard to guess.
Would monitor placement, vertical vs horizontal and corresponding Windows 10/video card settings make a difference?
Good to know your solution was to move the menu bar to a separate 5th display and it works correctly.
With my current 3 screen horizontal set-up using 3 34-inch 2560x1080, all from Matrix GForce GTX 1070ti…I had all kinds of initial problems setting them up properly. It took a few days, and finally a customer service call and the 1st tier support person said “are you sure you are using the included HDMI cables?” I thought why open up new HDMI cables when I have ones already in place? As it turned out I learned a lesson the hard way…there are different versions of HDMI cables. I’m not used to cable versions when the pins are identical 
On one of my older systems.
Windows 7 64. 4 identical 18 inch monitors side-by-side set up 1280x1024. Gforce 750ti? I cant remember for sure.
They worked almost flawlessly for years. The one issue is that I was forced to plug 3 of the 4 into my GForce, and the remaining one into the motherboard video out. Intermittently I would get screens in the wrong order. The solution was to unplug, plug back in and it would usually correct itself. My PC builder said he didn’t think it was a good idea to use both motherboard video out and GForce outs. I think he is right.