Let me first correct layouts/joomla/edit/params.php for the [1] part as it is wrong
then, after merge, let's discuss this here.
To help, please suggest the code to add IN CORE to create the situation you describe.

Here is the PR to test
#30435

Thank you!

To help, please suggest the code to add IN CORE to create the situation you describe.

@obuisard

What I am suggesting:

// Directly add a fieldset if we have no children
if (!$hasChildren)
{
...
} else {
    // Include the description when available
    if (isset($fieldSet->description) && trim($fieldSet->description))
    {
        echo '<div class="alert alert-info">';
        echo '<span class="fas fa-info-circle" aria-hidden="true"></span><span class="sr-only">' . Text::_('INFO') . '</span> ';
        echo Text::_($fieldSet->description);
        echo '</div>';
    }
}

Looks like you posted 2 issues for the same problem.
I close this one and keep #30433

Sorry, I misunderstood.
Both issues are different. One is for the top fieldset (which result in tabs) and one is for fieldsets inside a tab.
I created 2 posts because I assumed one suggestion might be accepted but not the other.

I am confused. Sorry. Reopening.

I cannot find a use case in the core, but I do have the issue in third-party extensions.
In the following example, this third-party extension has a description for the tab 'Picture' (top-most fieldset) but it does not show if there are several children fieldsets inside the tab.
The fix would allow the description to show while it does not now.

To test in the core:

• open the mod_articles_category.xml
• in the fieldset 'filtering' separate the fields by putting them inside 2 fieldsets
• add a description attribute to the fieldset 'filtering'

<fieldset name="filtering" label="MOD_ARTICLES_CATEGORY_FIELD_GROUP_FILTERING_LABEL" description="Top most description">

<fieldset name="filtering_child1" label="Child 1">
[some fields]
</fieldset>
<fieldset name="filtering_child2" label="Child 2">
[the remaining fields]
</fieldset>

</fieldset>

Without the updated code, the description won't show. With the updated code, it will.

I am confused. Sorry. Reopening.

Sorry about the confusion, I hope it is more clear now.

I combined both #30433 and here
Please test
#30450

closing