OK, I found the culprit. The ISO had a minor fault, Windows installed from it is working but the WinPE repair console is missing.
I noticed that using my install ISO, a command prompt flashed fast just before showing the input language and time / currency formats dialog. I had to capture the boot on a vm to video, then freeze the video to be able to, have enough time to read the message shown in this command prompt:
Further, after the command prompt the dialog was shown with wrong background colour, white:
Inspecting this a bit more, the two missing files are parts of the WinPE repair console. However, I have Windows installed using this ISO on three machines, all of them working perfectly except the one I today managed to screw up the EFI system partition (long story...). In other words this ISO although missing the repair console had no other flaws, Windows installed from it works perfectly
Anyway, I used the same ESD file and re-made the ISO now and did a test install on Hyper-V vm using the new ISO. This time the dialog background colour is blue as it should be, and the repair options is there:
(Compare the dialog blue background to screenshot above with with background.)