Why do clients disconnect when you add a new WLAN?

Making changes on the WLAN will cause the radio to reinitialize in order to obtain the new configuration. The radio reinitializes when adding a new WLAN or making any RF related changes to the WLAN.

On the Client Events in the Insights page, you will see an AP Deauthentication being sent to the clients when the changes are made. You can refer the audit log for the same.

Because of this behavior, we advise making WLAN changes on a site after business hours so that none of the services will get impacted.

Client events in the Insights page:

 

Audit logs showing this event: