I've had flickering issues with Plasma for a long time. The issue is certainly not specific to Solus. I've tested this with Kubuntu, KDE Neon (based on Ubuntu) and Netrunner. I also have a laptop (Thinkpad X1 Carbon) with a dock and external monitors.
Flickering is usually triggered by connecting to the dock, putting the laptop to sleep, waking it, and reconnecting to the dock. I don't notice any flickering when the laptop is docked and first booted.
I'm also using this dock with a MacBook, which doesn't have flickering, so I've ruled out the hardware. I don't have a Linux desktop to test the issues with, so I don't know if it's laptop specific. It does seem to be tied to sleep / wake and external monitors.