Back again with the Ender 3 Pro V4.2.2 board and ANTCLabs v3.1 BLTouch. I had got everything working just fine the other day, which is the bizarre part of my problem: suddenly I can’t get the BLTouch to recognize a flat surface.
When running G29 now , it shows my left side is approx -0.03, while my right side is +0.02. I have since levelled it manually with the knobs as a print is going and got it spot on perfect, but when running G29 right after that I get the same poor mesh reading. Further, when the exact same bed levelling print was done after levelling manually but using this poor mesh, the left side of the print is smashed in to the plate and the right side prints too high (as expected from the poor mesh). It seems that the BLTouch isn’t generating an appropriate mesh, but I can’t think of how or why this might be the case outside of the sensor itself is faulty.