This has happened more than once. When I travel near international border, TravelPass would sometimes kick in and I'd be charged $5. Yes I did turn off roaming per suggestion by customer service. I do need TravelPass as I spend 20-30 days in Canada per year. And it's hard to avoid traveling near the border when you live in Washington state.
I have the same issue when visiting family in Northern Vermont. Traveling to my grandson's high school which is 2 mi from the Canadian border, I find that I am pinging off of a Canadian tower and If I get a call or text, then my Travel Pass kicks in, if I have cell data turned on. Usually, I have data turned off but sometimes forget. I've begun setting Airplane Mode on when close to the border if I'm not expecting to need to use my phone. It is inconvenient, but it's just the way it is with coverage so close to the border.