My Android version is 4.4.4. 
Disabling background transfer stops the leakage, but app is not functioning when screen is locked(( so this is not a workaround for me.
By the way, TuneIn behaves in the same odd manner. However some less-known radio tuners dont.

Actually, stopping widget won't help neither. Traffic is still leaking. trying to suppress background traffic from Android system settings..