I use it with a Sony Alpha 6600, no problem. Test your device on a TV or PC screen with your HDMI cable to see if the problem persists. Is the HDMI cable of good quality and specified for 4K broadband or higher?

Hey!
I would check the monitor with other devices first. I own an S5 (first version) and have never had problems with this monitor, but I know a lot of people complain about “buggy” HDMI on the s5iix, i would test the camera with other monitors as well, so you can close out if it’s a monitor or a camera problem. (And if it’s canera than probably hope for a firmware update from Panasonic)
Hope your issues will be solved