Flask flash messaging is partially working

I have implemented a simple Flask application with login and register routes and I want to flash messages when username passwords are not matched. This is a part of my index and login routes,

@app.route('/')
@login_required
def index():
    return 'Hello World'

@app.route("/login", methods=["GET", "POST"])
def login():

    # Forget any user_id
    session.clear()

    if request.method == "POST":

        # Ensure username was submitted
        if not request.form.get("username"):
            flash("Must provide username")
            return redirect("/login")

        #And so on...

    if request.method == "GET":
        render_template("login.html")

The problem is that the flash message does not appear when I submit the form with blank inputs. Even so, the register route which has the same code renders the flash message without any error,

@app.route("/register", methods=["GET", "POST"])
def register():

    if request.method == "POST":

        # Ensure username was submitted
        if not request.form.get("username"):
            flash("Must provide username")
            return redirect("/register")
      
        #And so on...

    if request.method == "GET":
            render_template("register.html")

I have included Jinja templating in my templates. Can someone please point out why it is not working in only the login route?