Not tried a different cable, but was planning to order a longer displayport cable, so can try that tomorrow. I am currently using the displayport cable provided. I don't get the issue when using my work laptop through HDMI.I can notice very slight flickering with very back bagrounds but only when I work remotely via Citrix which often feeds less than 20 frames per seconds. I haven't noticed any flickering while gaming and definitely no line across the screen which seems to be far more annoying looking at the video you uploaded. The latter symptom is quite an issue I think, I think you might need to feed back this issue to iiyama. Have you tried a different cable by any chance?
Also, what GPU have you got?
I'm using the Sapphire Radeon RX 580 NITRO+.