Flask 405 Method Not Allowed - Despite Allowed - flask

Question

I am using Flask. I understand that one should specify the allowed methods via #app.route(<path>, methods=['GET', 'POST', ...]). However, even though I have "GET" in the methods list, I still get the 405 error:
The strange thing is that the "Request Method", i.e. "GET", is one of the methods in the "Allow" field in "Response Headers", shown above. How is this possible?
Here is my function for handling the route /session/<id>/admin:
#sessmgr.route('/session/<id>/admin', methods=["GET", "POST"])
def session_admin(id):
... # the details doesn't matter because they are not executed.
I am using the following form to submit the GET request
{% extends "base.fhtml" %}
{% block title %}Redirecting...{% endblock %}
{% block styles %}
{{super()}}
<link rel="stylesheet"
href="{{url_for('.static', filename='mystyle.css')}}">
{% endblock %}
{% block content %}
<div class="container h-100">
<div class="w-100 p-5">
<h7 class="text-center">Redirecting...</h7>
</div>
</div>
<form action="/session/1000/admin" method="GET" id="the-form">
<input type="submit" id="submit">
{% for key, value in args.items() %}
<input type="hidden" name="{{ key }}" value="{{ value }}">
{% endfor %}
</form>
<script>
$(document).ready(function() {
$("#the-form input[type=submit]").trigger("click");
});
</script>
{% endblock %}
The form is automatically submitted as soon as the page is loaded. However, I tried removing this automatic submission and manually click on the submit button, and the same thing happens. This doesn't make sense to me. Any ideas? Thanks!
Update
I have tried setting method='get' in the <form> but it didn't work.