I also tried playing around with xrandr output options, let me know if it helps:
1.
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xrandr --listmonitors
Monitors: 2
0: +*DisplayPort-0 3840/697x2160/392+3840+0 DisplayPort-0
1: +HDMI-A-1 3840/598x2160/336+0+0 HDMI-A-1
Tried turning off just the HDMI monitor - it wokeup by itself (same problem)
Now here's where things get different -
tried turning off just the DisplayPort monitor - it worked.
monitor went to sleep, while the HDMI monitor kept active (both were connected at the same time)
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xrandr --output DisplayPort-0 --off
So this time, monitor standby mode worked just for the displayport monitor,
while both screens were connected as long as just the displayport screen got the turnoff command.
2.
Another thing I noticed while trying
DisplayPort monitor has -VSync
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DisplayPort-0 connected primary 3840x2160+3840+0 (0x59) normal (normal left inverted right x axis y axis) 697mm x 392mm
3840x2160 (0x59) 533.250MHz +HSync -VSync *current +preferred
h: width 3840 start 3888 end 3920 total 4000 skew 0 clock 133.31KHz
v: height 2160 start 2163 end 2168 total 2222 clock 60.00Hz
HDMI monitor has +VSync
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HDMI-A-1 connected 3840x2160+0+0 (0x60) normal (normal left inverted right x axis y axis) 598mm x 336mm
1920x1080 (0x60) 148.500MHz +HSync +VSync *current +preferred
h: width 1920 start 2008 end 2052 total 2200 skew 0 clock 67.50KHz
v: height 1080 start 1084 end 1089 total 1125 clock 60.00Hz
Perhaps the issue has something to do with the +VSync versus the -VSync thing?
Does that help?
Edit: I set the HDMI monitor mode to the following:
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1920x1080 (0x7f) 170.000MHz +HSync -VSync *current
h: width 1920 start 1928 end 1960 total 2026 skew 0 clock 83.91KHz
v: height 1080 start 1105 end 1113 total 1119 clock 74.99Hz
with "-vsync", still problem persists. So probably not related?