The only problem I have had similar to these involves the Freesync. My monitors on both my Ryzen 7 2700, Radeon RX 480, with MSI mpg X570 Gaming Plus board and the Ryzen 9 3900X, Radeon RX 5700 XT, with MSI MEG ACE X570 board with Samsung Freesync monitors. With the Freesync enabled and you try to enter the BIOS you get a black screen. I honestly think the system goes to the BIOS, you just can't see anything. Disable the Freesync and the problem goes away. So I just live with it and enable the Freesync to run Super Resolution and if I need to enter the BIOS for whatever reason I turn turn Freesync OFF.
So just for curiosity sake, try disabling Freesync and see if that solves your problems.
A couple of the POSTS sound like they might be a power supply issue.