ok, after thinking through what I wrote, I have a working workaround:
created autostart.sh with
echo 1080p50hz > /sys/class/display/mode
echo 1080p60hz > /sys/class/display/mode
effectively letting the script do what I knew helped when done manually. interestingly enough it helps even though it is run before the HDMI connection is established. I don’t complain.
(I have tried to do only “echo 1080p60hz > /sys/class/display/mode” and unsurprisingly this did not work, because the system thinks it already outputs 1080p60hz and therefore doesn’t do anything.)
here is a dmesg with the workaround: http://ix.io/2Ts0
please note how on 0.455282@1 the system already wants to change to 1080p60hz but doesn’t because it thinks it already is there.
the workaround starts much later on 10.899305@1
and right on the next line it says:
[ 10.899411@1] fb: current vmode=576cvbs, cmd: 0x10000
so. this is not a solution but a workaround.
I will therefore not mark it as the solution and will reply and try to help with testing whatever ideas you might have. it is a regression from 9.2.5. (don’t know about the later 9.2. versions).