I downgraded to 9.2.5 and things went back to normal. Sorry, I did this before I saw the request to capture dispinfo.
Then I upgraded again to 9.2.6 – but this time, no problems – 4K UI working fine now.
FWIW here is my dispinfo from 9.2.5 and from my 2nd upgrade to 9.2.6:
9.2.5 dispinfo: http://ix.io/2QeU
9.2.6 dispinfo: http://ix.io/2Qf1
They seem identical (apart from CE version # of course).
My uneducated guess is that after playing a couple of movies for a couple of seconds, then stopping, to test 9.2.6, CE got somehow confused as to the display capabilities, and booted me back to a 1080p UI. A reboot didn’t fix it, the 4K options were simply not there in the options. I’ve tried to recreate this scenario now I’m back on 9.2.6 once again, but have not been able to do it, the UI stays 4K now, behaving nicely.