@boblui I wonder if you have the flight log handy that you took a screenshot of in post #5? What were the 4 motor speeds like at that moment when you had the abrupt yaw?
It just occurred to me that if one prop were to lose lift momentarily (micro pocket of turbulence if such a thing exists, prop strike, ESC glitch?!?!) then the diagonally opposite prop might slow down in sympathy to avoid flipping the drone, with the result that the 2 unaffected props would cause an uncommanded yaw. It's maybe a bit too contrived, as I'd have expected loss of lift in 1 corner to initially manifest itself with that corner losing altitude, ie drone pitching towards that corner - maybe another thing to check, if you have the log.