Flask项目中Bootstrap grid网格系统失效 页面布局堆叠问题

我之前使用了错误的Bootstrap版本，更换为Bootstrap 5.0的CDN链接后问题已解决，感谢Stack Overflow社区！

我是Flask、Bootstrap的新手，尝试了如下示例代码，但页面渲染后呈现堆叠布局（见第二张截图），而非Bootstrap官网展示的网格排布效果。

以下是我当前实际运行结果的截图：

以下是我的Flask项目配置代码，我已检查浏览器开发者模式，确认所有资源均成功加载，Bootstrap文件通过CDN链接引入，暂未定位到问题原因。
我也尝试过通过pip安装flask-bootstrap包并在app.py中导入，未解决问题。

app.py #

from flask import Flask, render_template

app = Flask(__name__)


@app.route("/")
def ooo():
    return render_template("ooo.html")

ooo.html #

{% extends "layout.html" %}
{% block content %}

<!-- Copied from bootstrap website -->
<div class="container">
    <div class="row">
        <div class="col">
            1 of 3
        </div>
        <div class="col-6">
            2 of 3 (wider)
        </div>
        <div class="col">
            3 of 3
        </div>
    </div>
    <div class="row">
        <div class="col">
            1 of 3
        </div>
        <div class="col-5">
            2 of 3 (wider)
        </div>
        <div class="col">
            3 of 3
        </div>
    </div>
</div>

{% endblock content %}

{% block script %}
    
    <script src="https://code.jquery.com/jquery-3.3.1.slim.min.js" 
    integrity="sha384-q8i/X+965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH+8abtTE1Pi6jizo" 
    crossorigin="anonymous"></script>
    
    <script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.6/umd/popper.min.js" 
    integrity="sha384-wHAiFfRlMFy6i5SRaxvfOCifBUQy1xHdJ/yoi7FRNXMRBu5WHdZYu1hA6ZOblgut" 
    crossorigin="anonymous"></script>
    
    <script src="https://stackpath.bootstrapcdn.com/bootstrap/4.2.1/js/bootstrap.min.js" 
    integrity="sha384-B0UglyR+jN6CkvvICOB2joaf5I4l3gm9GU6Hc1og6Ls7i6U/mkkaduKaBhlAXv9k" 
    crossorigin="anonymous"></script>    

{% endblock script %}

layout.html #

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <title>R10K OoO</title>
    
    <!-- Latest compiled and minified CSS -->
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" 
    integrity="sha384-BVYiiSIFeK1dGmJRAkycuHAHRg32OmUcww7on3RYdg4Va+PmSTsz/K68vbdEjh4u" 
    crossorigin="anonymous">

    <!-- Optional theme -->
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap-theme.min.css" 
    integrity="sha384-rHyoN1iRsVXV4nD0JutlnGaslCJuC7uwjduW9SVrLvRYooPp2bWYgmgJQIXwl/Sp" 
    crossorigin="anonymous">

</head>
<body>
    {% block content %}{% endblock %}

    {% block script %}{% endblock %}
</body>
</html>

感谢各位的帮助！

内容的提问来源于stack exchange，提问作者KHT