I have three Mikrotiks Groove GA-52HPacn and I have already had SXT G-5HPacD - the goal is to upgrade from N to AC on my wireless WDS MESH backbone. But AC failed to start in mixed enviroment with N only SXT G-5HPnD r2. After several tries I decided to make some experiments and turned off all N-only equipment - no luck; I put two Grooves on the table with distance of 1 meter, turn off all the comlicated thing, put one in AP mode and another one in Client mode - still no luck with AC. In all the described cases I tried to run in A/N/AC as well as in AC-only modes. But the maximum speed was 54. While turning the Grooves in N-only mode gives the stable speed of 150. Signal level in all the cases was not worth than -50dB, while noice was less -105dB, CCQ was always 80% or better. so probably physical conditions are not the issue.
Did you upgrade to the latest version?
Yes, Sir! 
that’s what I’ve started from: upgrade to the latest current, try RC, downgrade to bug fix branch, and back to current. And no luck in any case