Sorry for the late reply. 
 
What MR firmware are you running ? 
Were you able to capture the frame and confirm that the AP is really sending a frame with code '8' ?
Could the client be getting a code '23' , upon receving that code simply leaves the AP with code '8' ?
 
I think you should open a ticket and reproduce the issue and provide them a capture.
 
EDITv3 : This was solved under 28.7 : 
- When an Apple client enters an incorrect PSK the AP responds with a disassociation response instead of deauthentication response, resulting in multiple failed connection attempts from the client (Wi-Fi 5 Wave 2 APs)    
Might be worth a try to upgrade and test it !
 
Good luck