MT 3.9 at RB411. WLAN card is Sparklan WMIA-123AG (Atheros AR5213). Sometimes id disappear from the “interfaces”. There is no interface at the winbox and ssh console. Clients are registered and associated!!! - to “unknown” interface. After restarting the system - wlan is back, but only for few hours or few days, and problem is repeating. What is going on?
It is repeatable problem with RB411 and MT 3.9. I have the same problem with 3 pieces RB411 and MT 3.9.
can you tell us exact steps that need to be done to make the card disappear from the interfaces menu? we couldn’t repeat it here
We met the same problem with RB532 and SR5 Radio.
I cannot give a scientific explanation but…
we got this problem until we use UFL pigtail connectors.
Changing the pigtail connector to MMCX SOLVED THE PROBLEM.
We have a big statistic about 20 RB532 working H24 X 7 on field.
THIS is a completely different issue, and probably was caused by the UFL metal connector touching a part on the board, which made electrical short-circuit.
Sometmes, when i connect to RB411 via WinBox, then go to the Wireless - there is NO interface, and associated stations are connected to “unknown” interface!
I don’t know how to repeat, maybe there is hidden MT feature, like “wireless hide interface now” 
It was a joke:)
But problem is serious!
Exactly the same issue is still showing up in model 751 with firmware 5.25.
Anything is working and suddenly… wireless interface dissapears and an unknown is in the bridge. Customer can’t connect and if you reboot the system… it starts working again during a few hours until the same issue repeats.