I'm having this same problem, although it's somewhat different.
Phone is an LG Optimus L9 (aka LG P769), Android version is Jelly Bean 4.1.2., stock, not rooted, fully updated by T-Mobile U.S. Two phones of the same model.
When the phone is connected to the mobile network (T-Mobile U.S.) and an attempt is made to update the definitions when they're outdated - even though the software is set to automatically update, data is exchanged and it tells me that I have the latest version - which remains outdated. However, when I then set the phone to use WiFi with my local cable provider (Comcast) and repeat these same steps, the update comes in perfectly. This occurs on both phones, so it's not the phones.
As a test, I unchecked all three network types for updating, then when I selected to update, Avast still exchanged data - again telling me that I have the latest update, so at a minimum, the software appears to be ignoring the network settings.