Network Manager: GSM fails when WLAN is deactivated

As long as WLAN is enabled Network Manager will handle GSM (WWAN) connection mostly right.
When a GSM connection is successfully established and a WLAN connection is successfully established this leads to the issue: As soon as the WLAN connection is disabled in Network Manager GUI the connection GSM/WWAN ist distorted, stops operating, shows a completely erroneous status of trying to reconnect with a very high frequency (hundreds of times per minute).
Expected behaviour:
When a GSM connection is successfully established and a WLAN connection is successfully established it is possible to deacivate the WLAN without affecting the GSM/WWAN connection.