DVI should not control the monitor's actual physical controls - it does include a small non-display channel but IIRC that's used to get the display modes info from the monitor, and potentially to transmit contrast information and the like; some monitors will prevent you from adjusting contrast if DVI sends that info for example, but it certainly shouldn't disable the power button.
My guess would be a hardware issue - in the monitor itself - which is somehow triggered by the sequence in which you do enable the displays, and your system update being unrelated. It's a huge guess though. One thing to try is repeating both sequences (the one that locks your buttons and the one that doesn't) using a live CD - not a "nobara 38" one if such a thing exists, another distro. Trying both monitors on another computer would be an interesting test as well, although not necessarily that helpful (because if it doesn't occur there, it might just mean the issue is triggered by peculiarities in your graphic card).