Post request returning homepage for subset of users

Question

I've been using the same code for many years without issue and recently its not behaving as expected.

  $.post('/stripe/ajax/initialize_total_cost.php', {}, function (result) {
                 
                }, 'json').error(function (xhr) {
                    handleUserRegistrationError(data, xhr.responseText);

                });

When the above code is run, for a small subset of users, the xhr.responseText is the HTML from my homepage. I've spoken to a few of them and they're using the same setup as I am (Chrome on a Mac). In addition, I'm not sure if this has anything to do with it (doubtful but it's another piece), but the users affected seem to just be European customers.

To start my debugging process, I'm just hoping to understand if there are conditions in which a post request would return the HTML from my homepage.