I did go and try the other DVI port and had the same issues - as soon as the full AMD driver kicks in when booting Windows (during the transition from the blue window-logo splash screen to the full Win 10 login screen) the DVI ports stop working. The HDMI port outputs video all the way along.
Well, I have fixed the issue with the DVI ports apparently - no idea WHY it works, but it works now.
Someone on the Windows thread I started suggested I turn off Quick Boot in the BIOS. I had an option for 'Quick Boot' under the 'Advanced BIOS options' area that was enabled; I turned that off and sure enough, I am now able to boot into Windows just fine using the DVI output of my video card.
Why would that fix it, though? I didn't change BIOS settings between when it last worked and when it started to fail?
And I wonder if re-enabling it now would NOT break things. Yep! Turning it back ON still lets me see Windows load up with the DVI connector. I wonder if it was merely a matter of forcing the BIOS to save that caused something to correct itself, and not really the specific setting involved. Once corrected it would work from there forward.
I'll keep my fingers crossed.