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I'm getting exactly this issue. An ATI RX 480 on Kubuntu 18.04 with a ASUS MG28U on HDMI-1. It is on a 5m cable so using the displayport wasn't an option. The long HDMI cable is to allow the large system box to be away from my desk. I mostly use the system as a web and data server and it is stuffed with hard drives.
Works perfectly on 4.13 kernels. Fails on the 4.15 kernels before the sddm login screen.
4.15.0-43-generic <-- screen repeats HDMI no signal
4.15.0-42-generic <-- screen repeats HDMI no signal
4.15.0-35-generic <-- screen repeats HDMI no signal
4.13.0-36-generic <-- ok
On 4.13 kernels it runs the screen at 3840x2160 @ 30Hz. On 4.15 kernels it doesn't.
At a guess looking at the syslog, the 4.13 fell back to 30Hz at the hi-res while the 4.15 does not. Over the long cable the screen isn't detecting the screen at 60Hz.
Hope that helps. I may have to look at getting a longer active cable...