Hello
I recently upgraded APs in all my BS stations (6.38.7 → 6.40.9) and noticed that I lost possibility to disable HT RX Chains (I can uncheck chains but it does not work - see attached screenshots).
For me it was useful option because I’m living in very noisy area. In some of my locations - disabling more noisy polarity sometimes increase overall connection stability and increase possible DL speed my customers can achieve (decrase possible UL speed is not a problem, because DL/UL ratio I give to customers is about 4/1). On client side I have always enabled both chains by default because problem with noise apples mainly to BS locations (usually wide antenna pattern and high location).
I had to downgrade to 6.38.7 in case of at least 3 of my sectors where I just HAVE TO disable one chain for connection stability reasons… 
((
(APs do not have access to the Internet (and are in separate VLAN) so vulnerabilities are not a problem)
I have made more detailed tests in “sector” where only I and my neighbor are connected (so our dedicated “sector” is based on QRT 5 ac, 3 km away 
) - the problem exists also in 6.39.3 (bugfix). Latest bugfix release without this issue is 6.38.7
BTW: My config (except not_shown values):
/interface wireless
set [ find default-name=wlan1 ] amsdu-limit=3839 amsdu-threshold=3839 band=
5ghz-a/n/ac channel-width=20/40mhz-eC country=poland default-forwarding=no
disable-running-check=yes disabled=no frequency=5860 frequency-mode=
superchannel mode=ap-bridge rx-chains=0 scan-list=
default,(not_shown) security-profile=profile1 ssid=(not_shown)
wireless-protocol=nstreme wps-mode=disabled
/interface wireless nstreme
set wlan1 disable-csma=yes framer-limit=4000 framer-policy=best-fit
BTW2: I DO NOT want to upgrade my APs to the newest “stable”, I prefer “bugfix”
