I had a problem in my map with triangles and clipping getting messed up and spent friggin ages trying to find the problem. Eventually I started deleting brushes until the problem disappeared, and it only did so when I deleted a ceiling I had made using the polygon tool. The ceiling was a 12 sided poly with 32 unit border.
While I don’t know for 100% certainty if it was the poly tool at fault or something else that had happened, I figured it best to mention it just in case.
The problems caused were faces around the area (but not actually on the polygon brushwork) would randomly vanish, and some non drawing faces somehow became extended causing invisible clipping. It may have been a one off or something that had an effect on those brushes after they were created, this is just a heads up post 