I haven't seen any mention of mac os' other built-in wi-fi tools: http://www.letstalk-tech.com/access-wifi-scanner-macos-sierra/ I use these a lot and the scanner has more info than the command-line scanner and it's easily sortable. The Performance tool shows SNR with no need to install anything. Also, the iphone/iPad AirPort Utility scanner won't show wi-fi by default. It must be turned on under preferences.
I'm also curious how close the other APs are. Macs excel at picking out a crummy signal so with the 5 ghz power at max(assuming the others are too) there's a good chance it's picking a loud but poor far-off signal. We use 10 dbm on 5 ghz if it's meant for one medium-size room. I'm still experimenting with 16-14 dbm in the dorms so 5 ghz will easily service a neighbor. This is very general with a lot of exceptions.