I feel your pain, I’m experiencing something similar here:
RX 580 → HDMI @ FullHD → Marantz NR1504 → [EDID-Fake-Plug]
RX 580 → DP @ UHD → Dell P2715Q.

Sometimes, when I turn the monitor off and on again, my desktop resolution is
reduced to 2048x1536 (this is not even 16:9) and I have no other option than
rebooting to get 3840x2160 again. This was new coming with the DC patches but
does not happen every time I turn the monitor off and on again, rather rarely,
but is endlessly annoying.