It was already working until version 3.23, If you use version 3.20 you can scan wireless via API,
you can try with API command "/interface/wireless/scan","=.id=wlan1"
But this version's API service have some simple bugs specially on API request returns. If I remember right, there was a problem for ethernet status monitoring function and it was returning always !re.
A lot of bugs seems corrected on version 3.23 and 3.24 but now there is same problem on /interface/wireless/scan function this time.
I think that developers are missing some things while they are upgrading os.