Just noticed this on the current snapshot - if 40MHz is selected in wireless settings in Luci, then 'HT40' is passed to iw to set the configuration. However, iw expects 'HT40+' or 'HT40-' and not just 'HT40'.
daemon.notice netifd: radio2 (3426): Usage: iw [options] dev <devname> set channel <channel> [NOHT|HT20|HT40+|HT40-|5MHz|10MHz|80MHz]
...
user.notice mac80211: Failed command: iw dev wlan1 set channel 157 HT40
The plus or minus is automatic now. The proper setting is "HT40". The base channel may need to be changed to meet the band plan.
Now means which version? Should it work properly in 18.06.3 ? Thank you!
Be aware that OpenWrt will fall back to HT20 if it encounters a congested channel, HT40 is only possible if you have 6 consecutive channels without interference at your disposal (basically only if you're literally living in the sticks).