JavaScript input validation - unexpected results

I have this event listener function

document.addEventListener('input', function (e) {
    if (e.target.classList.contains('mileage-validation')) {
        if (document.getElementById('warningMileage').value > document.getElementById('scheduledMileage').value) {                
            document.getElementById('mileageError').classList.toggle('hidden');
        }
        else {
            document.getElementById('mileageError').classList.toggle('hidden');
        }
    }
})

with the following markup in a razor view

<li>@Html.LabelFor(m => m.Maintenance.ScheduledMileage) @Html.EditorFor(m => m.Maintenance.ScheduledMileage, new { htmlAttributes = new { @class = "form-control mileage-validation", id = "scheduledMileage" } })</li>
            <li>@Html.ValidationMessageFor(m => m.Maintenance.ScheduledMileage, null, new { @class = "validation-error" })</li>

            <li>@Html.LabelFor(m => m.Maintenance.WarningMileage) @Html.EditorFor(m => m.Maintenance.WarningMileage, new { htmlAttributes = new { @class = "form-control mileage-validation", id = "warningMileage" } })</li>
            <li>
                @Html.ValidationMessageFor(m => m.Maintenance.WarningMileage, null, new { @class = "validation-error" })
                <span class="validation-error hidden" id="mileageError">Warning Mileage should be less than Scheduled Mileage.</span>
            </li>

I'm trying to front end validate that the WarningMileage value is less than the ScheduledMileage value (displaying an error if false) before the user submits.

1. Test - input 10 in Scheduled Mileage field with 0 in Warning Mileage field Result - error is display

   wrong

2. Test - input 9 in Warning Mileage field with 10 still in Scheduled Mileage field Result - error is hidden

   correct

3. Test - Go back to Scheduled Mileage field and increase value to 11 Result - error is displayed (but warning mileage is less than scheduled mileage!)

   wrong

Hope this makes sense, no doubt it's something silly. Thanks in advance!