Visual glitch when dragging windows across screens

Hi all, I'm currently using a dual-screen Mid-2010 6-core cMP 5,1 with MacOs 10.13.6 and a Radeon RX 580 (Metal compatible, fully shows boot process). Boot ROM is 144.0.0.0.0.

Since swapping the graphic card from the old Radeon HD 5770 to the new RX 580, I noticed that when I drag a window from one screen to the other, some texts and labels get messed up: when modev to the other screen (no matter which one) they show a sometimes black, sometimes white, sometimes grey background. This happens especially (I'd say "almost exclusively") in Finder, Safari or Apple-made, system-related windows (e.g. the "Info on this Mac" window). Usually the glitch appears in editable or selectable text.

Other third-party apps (Photoshop, Chrome, etc.) never show any glitch, as far as I remember or can test.

Here are a couple of examples:

This is Safari: notice the URL bar and the weird all-black bar which shows in place of the Google search bar:

Note that the affected windows are purely "visually impaired": the underlying elements keep working, and the glitch disappears as soon as the mouse overs or clicks on the affected element.

Can someone explain this erratic behaviour, and tell me why only certain softwares are affected?

Thanks in advance!