Why does url_for duplicate url_prefix for blueprint when running flask via fcgi?

Question

Python

Good afternoon. There is a simple Flask application:
api.py

from flask import Blueprint
from flask import url_for, jsonify

api = Blueprint('api', __name__)

@api.route('/', methods=['GET', 'OPTIONS'])
def index():
    links = dict()
    links['docs'] = url_for('.docs')
    return jsonify(_links=links)


@api.route('/docs/', methods=['GET'])
def docs():
    doc = {
        " key": "value"
    }
    return jsonify(doc), 200

app.py

from flask import Flask, request
from .api import api

app = Flask(__name__)
app.register_blueprint(api, url_prefix='/api')

If you run the application via app.run()

from .app import app
if __name__ == '__main__':
    app.run(debug=True)

then when I make an /api/ request, I receive a resource with a normal link /api/docs/
in response. If I run the application on the server via WSGI:

from .app import app
from flup.server.fcgi import WSGIServer
if __name__ == '__main__':
    WSGIServer(app, bindAddress='/tmp/fcgi.sock').run()

when I make an /api/ request, I receive a resource with a link /api/api/docs/ in response .
Where does the duplicate come from? With what it can be connected and how to treat?