Does your punting phone have wifi, if you let it connect to the same router as the civvy phone at the same time, FB will know soon enough that they belong to the same person, even though you have not directly accessed FB from the punting phone.
Or are you a BT broadband customer, and did you have your civvy phone on the punt, if the WG also had BT , possibly your civvy phone connected to the internet while you were punting, so you were both on her IP address at the same time.
I have 2 civvie FB accounts, but not registered a phone number on either, and I have only ever searched WG phone numbers in google, rarly use FB on my phone. I pretty much only use Firefox, but a few months ago used google chrome for some reason to open my hotmail, and chrome raided my address book and all of a sudden FB is asking me if I knew about a 15 year back catalogue of girls
I have only used my punting after my first 4 punts, never saved anything with those early punts.
I'm going to switch my Nokia basic phone this year to a smart phone, then create a punting FB, punting whatsapp, using my existing punting email. I won't be connecting via my home broadband though, I'll stick to using mobile data to make sure there is no overlap