Details #title is being escaped / stripped incorrectly

Hi. Looked into this during Drupal South 2023 code sprint, but as previsouly commented we need more information.
Not sure in what context this happens, so I tried to reproduce within a module on 11.0-dev (in custom block configuration and build, as well as in configuration forms), but it always worked as expected (HTML tags stick when using #markup).

I'll have a try again with Drupal 10.1! brb

Thank you very much for taking a look! I just checked this in Drupal 10.0.9 (related code is equal to 11.x currently) and it still doesn't work.

I tried the implementation from the issue summary and from #4 / #5 and in both cases the content was removed!

With just text ("TITLE TEST") in #title the result was:

<li class="horizontal-tab-button horizontal-tab-button-0 first selected" tabindex="-1" data-horizontaltabbutton="0">
  <a href="#edit-field-paragraph-settings-0-style-spacing--fZq0LuuXqC4">
    <strong>TITLE TEST</strong>
    <span class="summary"></span>
    <span id="active-horizontal-tab" class="visually-hidden">(aktiver Reiter)</span>
  </a>
</li>

With HTML in the title ("<i>TITLE TEST</i>") in #title the result was:

<li class="horizontal-tab-button horizontal-tab-button-0 first selected" tabindex="-1" data-horizontaltabbutton="0">
  <a href="#edit-field-paragraph-settings-0-style-spacing--fZq0LuuXqC4">
    <strong></strong>
    <span class="summary"></span>
    <span id="active-horizontal-tab" class="visually-hidden">(aktiver Reiter)</span>
  </a>
</li>

(note the empty <strong></strong>)
Both the same using just #title or #title with #markup within (#4 / #5)

My suggestion is, that the HTML is not stripped in PHP, but in the JavaScript when building the tabs.

Finally I found the important missing piece -.-

$element['style'] = [
      '#type' => 'horizontal_tabs',
      '#theme_wrappers' => ['horizontal_tabs'],
      '#title' => $this->t('Style'),
      '#tree' => TRUE,
    ];

(added this to the IS now!)

So it's not a core issue, but a field_group module issue and already reported for various field_group types!
So I created a meta issue to sum that up and get things fixed at field_group: 🐛 [META] Fix HTML escaping issues Active

Closing this as duplicate, thank you all so much and sorry.

Moving the issue over to Field Group where it belongs.