This is how Mikrotik works with some devices. Normis confirmed it: http://forum.mikrotik.com/t/xiaomi-phone-low-wifi-tx-rate/93622/1
Probably they will not fix it soon (it is low priority). You shouldn’t buy Mikrotik as AP for non-Mikrotik devices. Actually I don’t understand why devices such as wAP or cAP exists and who buys them if they have such problems in 802.11 compatibility. Consider using unifi or some enterprise equipment.