You may see DV360 rejecting 1x1 tags, saying that the CM360 tag is invalid, like this:

Fake news.  It’s not a real rejection.

In their own documentation, Google notes “If the creative is a 1x1 and only meant for tracking, you can ignore this rejection”

If you see rejections on 1x1s from CM360 in DV360 due to the tag being invalid, fret not.