Hi! Just want to add that I´m also having these problems, and this has been through several versions of VWX and computers. Currently on 2018 SP2 and brand new Mac Book Pro 15" with best available graphics.
For now I only need to close the file and re-open to solve the problem, but it appears again after a few renders.
A new render issue came up today, where textures with glow image (on video or LED screens in Spotlight) doesn´t render correct. A white text on blue background appears as distorted black instead. There where glitches with other colors to. No picture of this at moment but will post if it happens again.