I think is is something related to the Intel NUC that I am using, since all my other machines, do not have the issue, and it also occurred when this machine was running Ubuntu.
Initially it was after a resume that it fell back to mirrored, but now it happens a few times after resume and them becomes stable.
I put a button on my panel to run a script that puts it back, it is just the xrandr command.
xrandr --output DP1 --mode 1920x1080 --rate 60.00 --pos 0x0 --rotate normal --output DP2 --mode 1920x1080 --rate 60.00 --pos 1920x0 --rotate normal
Some times I have to run it a few times as it keeps going back to mirrored until things become stable. This has now been going on for over a year.
I have gone up through all the kernels and am now on 5.10.