Python Jsonify returns text/html

I have a python application which returns a bunch of league fixtures:

class Fixture:
    def __init__(self, matchday, date, home_team, away_team, predicted_winner, spread):
        self.matchday = matchday
        self.date = date
        self.home_team = home_team
        self.away_team = away_team
        self.predicted_winner = predicted_winner
        self.spread = spread

import json


class League:
    def __init__(self, name, fixtures):
        self.name = name
        self.fixtures = fixtures

    def to_dict(self):
        return {
            "name": self.name,
            "fixtures": [fixture.to_dict() for fixture in self.fixtures]
        }

class LeagueEncoder(json.JSONEncoder):
    def default(self, obj):
        if isinstance(obj, League):
            return {
                "name": obj.name,
                "fixtures": [fixture.__dict__ for fixture in obj.fixtures]
            }
        return super().default(obj)

These objects are returned from a Flask API using jsonify:

from flask import Flask, Response, jsonify
from services import results_calculator

app = Flask(__name__)

@app.get('/results/<date_from>/<date_to>')
def get_results(date_from, date_to):
    fixture_results = results_calculator.calculate_results(date_from, date_to)
    return Response(jsonify(fixture_results), mimetype='application/json')

if __name__ == '__main__':
  app.run()

I have a Java application which invokes this endpoint:

List<LeagueFixtures> fixtures = webClient
    .get()
    .uri(requestUri)
    .accept(MediaType.valueOf("application/json"))
    .retrieve()
    .bodyToMono(List.class)
    .block();

import lombok.AllArgsConstructor;
import lombok.Builder;
import lombok.Data;
import lombok.NoArgsConstructor;

import java.util.List;

@Data
@AllArgsConstructor
@NoArgsConstructor
@Builder
public class LeagueFixtures {
    private String name;
    private List<Fixture> fixtures;
}

package com.footballpredictor.predictoragent.model.input;

import lombok.*;

@Data
@AllArgsConstructor
@NoArgsConstructor
@Builder
public class Fixture {
    private String date;
    private Long matchday;
    private String home_team;
    private String away_team;
    private String predicted_winner;
    private Double spread;
}

However when the code hits, I get the following error:

org.springframework.web.reactive.function.UnsupportedMediaTypeException: Content type 'text/html;charset=utf-8' not supported for bodyType=java.util.List<?> at org.springframework.web.reactive.function.BodyExtractors.readWithMessageReaders(BodyExtractors.java:205) ~[spring-webflux-6.0.10.jar:6.0.10] Suppressed: reactor.core.publisher.FluxOnAssembly$OnAssemblyException: Error has been observed at the following site(s): *__checkpoint ⇢ Body from GET http://localhost:5000/results/2023-06-02/2023-06-05 [DefaultClientResponse]

Am I missing something that is causing the jsonify function to be ignored?

In the flask API I have tried:

• Returning json.dumps(fixture_results, League_Encoder)
• Simply returning jsonify
• I tried the answer from here: Flask jsonify returns bytes and string instead of json object