hi the.sleeper
What's not showing in airwave but is fundamental to the issue here is that the APs are on 80MHz channels (which is indeed the default, so not necessarily anything that someone chose).
From the ARM stats, we can see that the APs are not just on ch 149, but
rather 149E which means 149-161 as 80MHz.
From AP Marple-AP-M3533-C351-2-142506
Interface Name :wifi0 (5GHz)
Cur Chan: cca_ibss/cca_obss/cca_intf:0/23/10
Bcn fail/Bstuck reset/Scan rej(l) :403642/0/407
Current ARM Assignment :149E/12.0 <<<<<
It also appears that due to your locale, there are only 2 x 80 MHz channels available, which are 36E and 149E:
HT/VHT Channel Summary
----------------------
Bandwidth Channel range Total interference index
--------- ------------- ------------------------
40MHz 149-153 139
40MHz 36-40 224
40MHz 157-161 138
40MHz 44-48 187
80MHz 149-161 277 <<<
80MHz 36-48 411 <<<
The next thing to consider is how ARM will select channels. The table above shows a resultant number, in the case of 80MHz for 36E it is 411, and for 149E it is 277. How those numbers are derived is a topic for another time, but the net effect is that the highest number is the least desirable, and ARM will choose to put a new AP on 149E rather than choose the other option, 36E which it views as worse.
Having said that, that may or may not be the correct decision, it depends on a few things, but let's get some basic questions out of the way first.
a) Does your regulatory domain allow you to use UNII-2 and UNII-2e channels (or, can you share what country that this operates in)
b) How heavy is the requirement of needing 80MHz ? if you were to drop to 40MHz channels would anyone i) notice ii) care ?
c) how many APs are neighbouring around these 3 APs, e.g. how dense is this (office ? educational ?) - any other networks around (e.g. floor above is a different company etc.)