Hi again Richard,
It's getting increasingly peculiar then.
The EDID received may be different.
Would you please be able to connect the monitor in a way making it work at 4Kp60 and use a utility to capture the EDID you get?
If so, the utlity I could recommend is
DumpEDID.
DumpEDID.exe > c:\temp\edid.txt gives a text file with the output I could look at.
If you need more details on how to run that, do not hesitate to ask.
Kind regards,
Alban