I am having the same issue and have test both 5.5 and 5.9 whenever I set datarates or set ack value on access point to anything other than dynamic clients drop.
Access point and clients are mikrotik running sr9 radio cards.
Upgraded a RB1200 running ospf, vlans , pppoe and a few eoip tunnels no known issues at this time traffic usually under 20 Mbps will report back if needed. Previous version was 5.7