I have reported this same issue in the Beta thread, starting wth 8.5.1RC - Getting too many False DFS hits.. only way to counter is to roll back to 8.5.1 Beta3. Had 5 AP's after upgrade and random times during the day would kick all the clients and lock on to 5.62Ghz and all the clients would drop except the new AC ones and it would generate support. I had this issue on both Mixed and FF mode.
Here is the stats
Upgraded 15 Ap's
10= FF (2) Constand DFS Hits
5= Mixed (3) Constant DFS hits
10 Seemed ok, however 5 of 15 having DFS problems let me no choice but to roll back.
not really, randomly on any DFS frequency,
When they reboot they appear fast then they stop doing it and for some reason they randomly appear new.
But it is not a specific frequency dfs always changes.
5500-5520, Seem to be the worse in every case... 5515 I get DFS events the worst on multipul APs 100's of miles from each other after the 8.5.1 upgrade... But I can also verify DFS hits are less intense on 5525-UP.
Hope that helps.
Thursday - last edited Thursday
I see that many people claim false hits from dfs with firmware 8.5 and 8.5.1.
I realized that when I restart an antenna with firmware 8.5 or 8.5.1 more dfs appear, and when I start an antenna in the vicinity of the aforementioned mentioned that also has 8.5 or 8.5.1 dfs events appear in these and in which they are in the emission range that have this firmware ..
2 hours ago
This is a known issue and so the fix didn't made it into AC2.1 final.
Some guys in this topic may have real (but false ) DFS hits.. but like i said... real ones are reported in device log too.
We all work for KUDOs here.