I have seen this on the MacBook as well as the iPhone.
Still bugs me at home reading twitter in the bath. Switch changes not helped.
I did the dump on all channels of the APs and showed the Apple device not trying to send anything.
Seems to be triggered by IPv6, and that means commonly FireBrick but not always by any means and a few people saying they have seen the same even not using Unifi!
So may be totally Apple borked.