Most likely, python-flask uses an old version of Swagger UI. Try using the latest dist assets from the Swagger UI repository.
I've tried to use the latest version 3.25.5 of Swagger UI by grabbing the dist files and following these steps and it didn't resolve it. I also tried cloning the whole repo, rebuilding the dist, and using those files, but that also did not resolve it.
This is the index.j2 I'm using per the answer and I'm using the generated code to setup the app:
app = connexion.App(__name__, specification_dir='./swagger/')
app.app.json_encoder = encoder.JSONEncoder
app.add_api('swagger.yaml', arguments={'title': 'test'}, pythonic_params=True)
app.run(port=8080)
