I'm not sure when this started but it's very recently and feels like after Radeon Settings was installed or at least the latest drivers. I can't be 100% sure...

Basically when in crossfire (can't reproduce with single card), the screen glitches out for a frame. It may be subtle, or it may completely cover the screen. Almost like part of the screen got repainted over and over again from top to bottom.

This specific issue does not happen in other games or benchmarks.

See it in action:

Witcher 3 Full Screen Glitching in Crossfire - YouTube