I have found a workaround:
Set “Settings” – “Location” – “Mode” to “Battery saving”.
So my mobile network is not lost when starting Google Maps. Downside: Some apps (like car navigation) will complain, and your phone is not usable for high accuracy location apps (like car navigation) anymore. With “High accuracy” or “Device only”, I lose mobile network when starting Google Maps.
Please note that when you are disabling and re-enabling location service, that setting will be reset to “High accuracy” and you will lose your mobile network again.