This is expected and normal. All the intelligence is in the CAPsMAN device, i.e. the 5009 in your case.
Welcome to the club. There is a very similar topic - http://forum.mikrotik.com/t/hap-ax3-random-wireless-disconnects/176477/1 . TL;DR - some people experience all sorts of disconnection issues with the wifi-qcom driver since upgrade above 7.14.3, while others claim they never had any. The seemingly quite popular Intel AX201 is often mentioned as an affected device, you may find it in your Dells too.
I was thinking I was safe as I only have wifi-qcom-ac at home, but two days ago my laptop started disconnecting - guess what my WiFi card is. So I’ve partially followed this advice: http://forum.mikrotik.com/t/hap-ax3-random-wireless-disconnects/176477/160 in terms that I have provisioned the physical interfaces with a hidden SSID that is not used by any client and attached both my “production” SSIDs to virtual interfaces; I did not set a different channel width to virtual interfaces as I don’t think it has any actual effect.
I don’t think fixed 5 GHz channels should cause any issue unless you choose frequencies that must give way to radar and they get hit by an actual radar or by some glitch of a WiFi device so they have to switch off power and wait for some minutes until they can start sending again. But if this happens, a dynamic frequency choice doesn’t help as when a new channel is chosen, it must first listen for said few minutes and only then it can start sending.
Regarding the manual setting of security.ft-mobility-domain, the manual states there is a default value, and as various devices do roam successfully as I move between the (overlapping) coverage areas, I did not feel any need to set it up.
According to the topic referred above, after a change, it usually takes days until the issue re-appears, so I don’t yell “hooray” until I can enjoy at least a week without a disconnection of my laptop.