I have this problem since a few driver versions back.
Basically whenever both monitors are enabled and I open a full screen app, both screens start flickering/glitching.
The fix I found for the moment is to disable FreeSync so I'm guessing it's a FreeSync bug.
GPU: Sapphire Radeon RX580 8GB [19.12.2-clean install]
MB: MSI B360M PRO-VDH
CPU: Intel i7-8700
RAM: 2x8GB HyperX 2400MHz
OS: Win10 v1809 (17763.379)
Monitors: LG 22MK430H connected via HDMI