That rings a loud bell with me. Doesn't that function search for a WiFi signal or mobile data connection that has an internet connection if the current one (i.e. the BV) doesn't? I found this on an old phone of mine (S3 I think). I fed this back to Pittasoft (UK) and they put it up as a fix on their site.
What would happen is that when I joined the BV WiFi, the phone would find no internet and disconnect giving a misleading error message. I had to turn off, as you say, something like 'Smart network'. (can't remember the exact setting name).
If this is the case, it is possible that on the OP's phone, the lack of an internet connection from the BV is causing the phone to switch to mobile data to get one.