Hello,
I recently upgraded my MT to 2.9.8 from 2.9.6.
Forgot, it’s a RB532.
I’ve a fairly clean setup with a single 2.4GHz Atheros AR5213 card. I’m running ap-bridge mode with a WPA access on the main interface and a virtual-ap in WEP mode.
Now the issue..
the main WPA enabled WiFi worked just fine, it was more stable then with the old 2.9.6 release. But the virtual-ap wep enabled WiFi just didn’t work.
The message “extensive data loss” keeped apparing in the loggs and the client could not connect.
I tried several things, like disabling-enabling the card to resetting the configuration and redo everything from scratch, all with the same result.
I then downgraded to 2.9.7 .. same result.
Now I downgraded to 2.9.6 and it all works as it should again!
Something is fishy with RouterOS after 2.9.7 on the 2.4GHz band.
Well, thought I’d share my experience with 2.9.{7,8} with the masses 
//paddy - happy MT owner