So when we were testing a RADIUS config with a client in our lab, we noted the following happen. A client device turns off its radio. Even though the client radio was turned off, the client entry remained on the WLC/radio for times between 5 minutes - 34 minutes. So three questions.
Session timeout set to 60 secs is expected to cause roaming issues and random wireless disconnections, set it to the maximum possible value as per your WLC (Catalyst - 86400, Aironet - 65535 or 0)
What you need to check is idle timeout, check what is the value configured there. Also if you are using LWA or CWA you need to take care of sleeping client timeout as well.
Recommendation is that you always set a higher session timeout and make sure idle timeout is always lower than the session timeout. Also for better understanding next time please post the platform, AP model, auth type, ssid switching method etc.