So... it's been nine months and multiple releases, but this PITA is still negatively impacting my sanity. My stock agency wants a zillion keywords per image, so having a majority of my entries auto completed WRONG burns up a lot of whatever lifetime I have left.
Updates to Camera Raw, Lightroom Classic, and the Lightroom Ecosystem were released yesterday evening and contain a fix for this issue. Please install the appropriate update and see if it solves this problem.