I have noticed a bit of a strange situation with one of my routers... Maybe this is by design.
Hardware is a Mikrotik HAP AC2 running QCA4019 chipset. OpenWRT 21 and now 22. Upgrading from 21 to 22 did not resolve this.
So, its a dual band router where wlan0 = 2.4g, wlan1 = 5g.
As is usually the case my 5g band is not frequently used as its just the laptops and phones.
What tends to happen is once the 5g band goes from "in use" (1 or more client) to "idle" (0 clients), the 5g interface (wlan1) will reset. logread will post a fairly quick "enabled -> disabled", "disabled -> enabled". And right after wlan0 will reset as well, all clients on wlan0 will deauth and reauth.
This all takes place fairly quickly but none the less, its a bit annoying as I have a fairly large IoT network here and even a few seconds of disconnection can mean my significant other "...the lights in the kitchen don't work!" 
I will add this can sometimes take a while to take place. It's not instant, but within some amount of time (XX minutes?) this will take place.
I will add another piece of information : On the 2.4g band I have 1 WDS client who is also running OpenWRT. When I look at the logs on this client it deauths with "Reason: 3=DEAUTH_LEAVING", so I believe the AP signals to the clients "hey, I am about to disconnect you all... disconnect or be disconnected".
My guess is the interface going to zero connected clients signals to the firmware / driver or hostapd to either change a power saving mode or some other (antenna?) mode, which causes the driver to reset all interfaces since they are on a single chipset. Another possiblity is that wlan1 resets wlan0 since OpenWRT (since a fairly recent release) runs all interfaces on a single hostapd instance?
Any ideas how to overcome this?
EDIT : Just to be 100% sure I have just disabled my 5ghz band and will see if this is in fact the cause. I am fairly certain it is as the logs always show wlan1 (5g) enable -> disable first, followed by wlan0 (2.4g) right after.