What version of MacOS are you currently using on your MacBook? Earlier versions of Mojave had graphical glitches that were fixed in later updates. It's possible that this could be causing the issue.
Also, I'd say that some sort of graphical glitch happens on occasion when I connect my 2017 13" MacBook Pro to my two external monitors, but it is more of a distorted image as the GPU resizes the desktop to fit my monitors. Nothing like what you are seeing though.