I'd bet my reputation that AT&T is bundling data packets across the border - and that this could be the basis of a big class action suit.
In my case, I was driving northward from Oak Harbor to Vancouver. Roaming was off on my AT&T phone - so there should have been no intl data billed until I turned it on for that one minute.
If pressed, AT&T would need to correlate three pieces of information - the clearing house data, their billing, and the canadian network provider's logs. Each entity is legally separate here - if all three logs line up, it's going to be hugely compelling evidence.
It's very unlikely that AT&T is gathering up your data charges and billing you as you go over the border - it would be supreme incompetence at a carrier of this scale. More likely, something's gone wrong at the clearing house, and you should press them for aligned, detailed records. (I audited telco billing systems for a few years.)
But, is it really worth $30? I'd just pay TBH.
OP sounded pretty sure, but I'm skeptical.