As nothings happened on this ticket for a few months, I've reached the task
again. I can confirm DisplayPort-0 remains connected according to xrandr when
its off, prior to the problem happening.

I'm now running Xorg with '-logverbose 255', but nothing more is logged when
the problem occurs.

A new failure mode has emerged - sometimes the middle Samsung monitor goes off,
and requires disabling and reenabling via the XFCE4 Display program to get it
back on again (haven't tried with xrandr etc).

I'm a bit disappointed nothing more is logged by X, I'm going to see what I can
find out in XFCE4's tooling.