Flexbox combined with vertical overflow is not working correctly

I have an issue that has been bugging me for hours and I just can figure out how to fix it.

I have created a card (similar to the ones in Bootstrap). I have given it a fixed height. That's all fine.

However, inside the card. I have a div, with a class called "transaction-lines". In it, you have several individual lines inside the div. There could be 1-50.

What I am trying to do is to get the overflow-y working, so that if the height of the "transaction-lines" div is bigger than the fixed height of the card, it will trigger the scroll mode (overflow-y: scroll).

If you look at it now, it doesn't trigger scroll mode correctly. If you remove a few comment lines, from the fiddle, it will look normal.

https://codepen.io/DocRow10/pen/MWQPLzo

<div class="test-card style-one main-details" style="height: 34vh;">
    <div class="card-main-content">
        <div class="card-main-content-container">
            <div class="content-container">
                <section class="tab-section">
                    <div class="transaction-lines">
                        <div class="line">
                            <div class="indicator">
                                <div></div>
                                <div></div>
                                <div></div>
                                <div></div>
                            </div>
                            <div>
                                Test Comment Line
                            </div>
                        </div>

                        <div class="line">
                            <div class="indicator">
                                <div></div>
                                <div></div>
                                <div></div>
                                <div></div>
                            </div>
                            <div>
                                Test Comment Line
                            </div>
                        </div>
                        <div class="line">
                            <div class="indicator">
                                <div></div>
                                <div></div>
                                <div></div>
                                <div></div>
                            </div>
                            <div>
                                Test Comment Line
                            </div>
                        </div>
                        <div class="line">
                            <div class="indicator">
                                <div></div>
                                <div></div>
                                <div></div>
                                <div></div>
                            </div>
                            <div>
                                Test Comment Line
                            </div>
                        </div>
                        <div class="line">
                            <div class="indicator">
                                <div></div>
                                <div></div>
                                <div></div>
                                <div></div>
                            </div>
                            <div>
                                Test Comment Line
                            </div>
                        </div>
                        <div class="line">
                            <div class="indicator">
                                <div></div>
                                <div></div>
                                <div></div>
                                <div></div>
                            </div>
                            <div>
                                Test Comment Line
                            </div>
                        </div>
                        <div class="line">
                            <div class="indicator">
                                <div></div>
                                <div></div>
                                <div></div>
                                <div></div>
                            </div>
                            <div>
                                Test Comment Line
                            </div>
                        </div>
                    </div>
                </section>
            </div>
        </div>
    </div>
    <div class="card-footer-container">
        <div class="card-footer">
            <button class="primary-button">Button</button>
            <button class="primary-button">Button</button><button class="primary-button">Button</button><button
                class="primary-button">Button</button><button class="primary-button">Button</button><button
                class="primary-button">Button</button><button class="primary-button">Button</button>
        </div>
    </div>
</div>

Any help would be appreciated.

Thanks.