Received unexpected DISCOVERY REQ, session tear down

My Forticloud managed 223C FortiAPs keep failing with this message, "Received unexpected DISCOVERY REQ, session tear down". Sometimes all of them in sequence, which is rather inconvenient. I have been removing things from the network to eliminate possible causes, but there is not much plugged in now. I was originally thinking it might be a chromecast. Fortinet Support has so far been unable to resolve the cause.

 

8 08-07 17:04 ap (ap-status) FP223C3X3 ap-fail Received unexpected DISCOVERY REQ, session tear down 9 08-07 17:03 ap (ap-status) FP223C3X2 ap-leave N/A 10 08-07 17:03 ap (ap-status) FP223C3X2 ap-fail Received unexpected DISCOVERY REQ, session tear down 11 08-07 17:03 ap (ap-status) FP223C3X1 ap-leave N/A 12 08-07 17:03 ap (ap-status) FP223C3X1 ap-fail Received unexpected DISCOVERY REQ, session tear down

 

Has anyone seen this before? Should an unexpected discovery request to cause the AP to fail? What might the cause be?