I remember a while ago this was caused by a firmware issue on the hub which was / is resolved by a firmware upgrade, if you have recenlty had the hub installed it may not yet have received the upgrade.
I have heard that this can also be caused by high downstream / upstream power levels on the line but to have this checked you would need to speak to VM technical support.
Welcome to the community. There are some versions of the HUB3 that can have a flashing green wifi light. It's something we are looking into but it doesn't appear to indicate any kind of fault with the connection.