Modifying instance data while ajax submit, reverts input data

I have the following method on my Vue instance:

    ajaxSubmit: function(event) {
        this.loader = true;

        var form = event.target;

        var action = form.getAttribute('action');
        var data = new FormData(form);
        var method = form.getAttribute('method').toUpperCase();

        this.$http({
            body: data,
            method: method,
            url: action
        }).then(function(response) {
            if (response.body.alerts) {
                response.body.alerts.forEach(function(alert) {
                    this.alerts.push(alert);
                }.bind(this));
            }

            this.loader = false;
        }, function(response) {
            // same as success callback
        });
    }

Let's say I have an input with a default value of "Test". If I update it to "Test111" and then submit. The value reverts to "Test". The submitted value is correct, as "Test111" gets saved to my database and also after a refresh, the input has the correct value.

I managed to narrow down the problem. I observed that it only happens if in the ajaxSubmit method I change data on the instance. For example if I delete the lines:

this.loader = true;
this.loader = false;
this.alerts.push(alert);

everything works as expected.

Also, if an input is bound to the instance via v-model, it doesn't revert its value.

So, in the end I guess I found the source of the problem, but I don't understand it, and I can't solve it.

Any help would be greatly appreciated!