@WSI 

I assume that those clients showing on controller are getting weak wifi signal from the EAP but still stick to the EAP's Wi-Fi. They didn't switch to other available and working Wi-Fi automatically though they failed to obtain IP addresses for connections. So they are not actually connected. Perhaps it could help if you set RSSI thresholds to force disconnecting the clients with weak signals. 

@Yannie 

It's an office building. Those clients that show connected after a working day for example are not in the building or close at all. They just don't disconnect from the controller.

It's not happening to all clients. Let's say 100 clients are connected at the beginning of the day and at the and 20 are stuck at the end. Some devices have a multiple days uptime however they go home every day.