Yep, you are correct. It looks like when adding this yourself, you didnt include p tags, while when adding it with the details insertion tool, it will do that.
Yours:
<details dir="ltr" open="">
<summary>
This is a HTML details
</summary>
Lorem ipsum dolor sit amet, consectetur adipiscing elit. In et ultricies ante. Fusce vel nisi tristique, tristique urna ut, dapibus lacus. Nulla dignissim quis ex a interdum. Integer nisl nisl, egestas in efficitur a, gravida vitae odio. Proin ut velit sit amet quam malesuada aliquet. Cras tempus in risus vel lacinia. Vivamus ut luctus nunc. Duis risus velit, molestie non lectus vitae, ornare gravida elit.
</details>
Insertion Tools:
<details dir="ltr" open="">
<summary>Summary</summary>
<p>This text will be hidden</p>
</details>
The p tag added by the insertion tool keeps the text on a new line.