Then it looks like you’ve done an exhaustive set of tests. Unless someone sees something I missed, this points to a defective AP board. I could not find the part number on the Bambu Site and there may be revision issues to be considered. So if you haven’t already done so check with support. Who knows, they may offer you a complimentary board.
What you might try is to simply reseat the WIFI antenna connector. Here’s the wiki on how to replace the AP board which should help you find where the connector is. If you haven’t done this before, I recommend a sturdy set of tweezers and grab the connector from the sides, not by the wire. The connector is sturdy but its weakness is in the part where the wire crimps into the connector so be careful.
Aside form the link I posted above for the antenna, here is a detailed link for the AP board that may make it easier to access the antenna.
Good luck🍀and let us know what Bambu Support tells you and what your final outcome is.