Clients are getting randomly disconnected from Wifi FAP-U431F on WLC 50D

[alaaa]

Hi Everyone, 

we have a complete Fortinet stack in our network

Fortigate 101F Running on v6.2.4 build1112 (HA, passive-active)

FortiSwitch 448D running on v6.4.1,build0411

FortiWLC 50D Running on 8.5-2build-5

13x FAP-U431F 8.5-1build-16-patch

the disconnections can be categorized into 2 issues:

1. few APs will drop multiple clients at the same time and after 10 min the WLC will show high AP memory usage.

and during this disconnection, the WLC will become unsingable form the wired network.

2. Clients are getting randomly disconnected.

We have observed that TX will somehow freeze sometimes

and clients will be connected but without internet for a minute or 2. it reconnects automatically after 2-3 mins but user can reestablish the connection to the SSID to make it work.

clients are mostly using AX band, we have disabled it on the WLC to get better results and performance. we also tried the latest intel driver 21.120 and the killer AX 1650 driver and changing the roaming aggressiveness and MIMO power saving options but without luck.

we have replanned it many times with ARRP and turned off Auto Power but without any luck

and the latency to the default gateway from this SSID is always between 6 and 15 ms.

I've tried provisioning one AP directly to the Fortigate and the default gateway ping immediately got better (1ms)and never experience any disconnections and WiFi 6 works.

did anyone experience the same? 

and any advice?