Irritating Problems Using Multiple Monitors on a Mac Pro

I have a late 2013 Mac Pro running Mac OS Mojave 10.14.6, attached to which are three identical Dell PH-K321 4K monitors. These three monitors are connected directer into three of the six available Thunderbolt slots offered on the back of the Mac Pro. My complaint is not that the monitors do not work; they do, with crystal clear precision. (I have one monitor turned vertical in the center flanked by two horizontal with all three sharing the same horizontal axis; a way-groovy set-up.) My grumble is that after waking up from sleep or restarting, the applications that I had running invariably relocate themselves to other monitors, all needing relocating and re-proportioning to make sense of everything again. The most irritating problem is that when one application prompts me with a dialogue box, it appears on another monitor, and more often than note, hidden under another application, forcing me to hide all the other applications just to find it.

I have rifled through the Apple FAQs on the matter of multiple monitors without finding a similar problem, and Dell's technical support websites never seemed to have heard of Macintosh computers, let alone have drivers to support their products, so I lay my problem at the feet of the true experts, real customers who use Apple products to their fullest.

My thanks in advance!