Tortilla is absolutely flat bread.

We label foods based upon the number of carb sides.

Open face sandwiches are not a thing. A sandwich requires two non-contiguous opposed bread layers, otherwise it isn’t a sandwich. It’s a pie. A bottom carb layer with a top layer of substance. Pizza, pie, “open face sandwiches”, and French parfait are all pies.

Now, by having only one contiguous bread, but covering three sides, that would put tacos, hotdogs, Philly cheese steaks, and subs all in their own category (arguably a misshapen pie, but that’s a deeper debate).