重写FlaskForm.validate()不会让POST请求通过

from flask_wtf import FlaskForm
from wtforms import StringField, PasswordField, SubmitField
from wtforms.validators import DataRequired, Length

class LoginForm(FlaskForm):
    username = StringField('Username', validators=[DataRequired(), Length(min=4, max=20)])
    password = PasswordField('Password', validators=[DataRequired()])
    submit = SubmitField('Login')

    def validate(self):
        if not super().validate():
            return False

        # 自定义验证逻辑
        if self.username.data == 'admin' and self.password.data == 'password':
            return True
        else:
            self.username.errors.append('Invalid username or password.')
            return False