I eventually brought in my Meraki sales representative.
In my situation, Meraki believes the antenna is at fault, indicating to the AP the wrong antenna model. They intend to replace all 20 of the F6 antennas. I will hold off on forming an opinion until I try this solution, but I'll defer to their expertise for now.
I expect these replacements to arrive in a couple of weeks, and I will provide an update on the status after testing and installation.