@HLeithner can you please comment?

I can confirm this problem, maybe someone with better js knowledge then me could look at this.

Repeatable field is going away in Joomla 4. It will be replaced by the better version, called Subfields
See: #24711

Please test if the problem happens with this new version too and report in that PR.

We support 3.x for at least 2 years after j 4.0 release so this should be fixed.

closed for Reason stated above.

The DOM is changed when the editors are sorted, so the editors need to be rebuilt.

Firstly, you need an event to be triggered when the sort is completed, which you can then use to trigger the re-initialisation of the editors.

As an example, an "update" option can be added to the sortable options in media/system/js/subform-repeatable.js to trigger a "subform-row-sort" :

See https://github.com/joomla/joomla-cms/blob/staging/media/system/js/subform-repeatable-uncompressed.js#L52-L58

// bind move button
if (this.options.btMove) {
    this.$containerRows.sortable({
        items: this.options.repeatableElement,
        handle: this.options.btMove,
        tolerance: 'pointer',
        update: function (event, ui) {
            var $row = ui.item[0];
            self.$container.trigger('subform-row-sort', $row);
        }
    });
}

then in media/editors/tinymce/js/tinymce.js, a listener triggers the editor setup after the sort :
See - https://github.com/joomla/joomla-cms/blob/staging/media/editors/tinymce/js/tinymce.js#L106-L115

jQuery(document).on('subform-row-sort', function (event, row) {
    Joomla.JoomlaTinyMCE.setupEditors(row);
});

but, the existing editor must first be removed before it is re-initialised, so adding a few lines to do this in the same file :

See - https://github.com/joomla/joomla-cms/blob/staging/media/editors/tinymce/js/tinymce.js#L59-L63

if (element) {
    // We already have the Target, so reset the selector and assign given element as target
    options.selector = null;
    options.target = element;

    var editor = tinymce.get(element.id);

    if (editor) {
        editor.remove();
        delete Joomla.editors.instances[element.id];
    }
}

The second part is for the core Tinymce only, but JCE will have it's own fix for this in the next update. The sortable event is required for all editors.

I will try and put this into a PR this coming week.

I can still confirm this bug on the current 3.9.19 release and tried to apply the above fix manually.
However it does not work...
Any way to fix this in the current release?

It is Browser behavior,
Explanation https://stackoverflow.com/a/2542043/1000711

In short: browser will clear iframe content when DOM changes,

@ryandemmer idea should work

Please test #34808 Joomla 3 fix.

However it still an issue for Joomla 4, so please keep it open for now.

@apophis1982 Could you test PR #34808 ? Thanks in advance.

@pixxelfactory Could you test PR #34808 ? Thanks in advance.

However it still an issue for Joomla 4, so please keep it open for now.

@Fedik Will you also make a fix for J4?

I have planed, it need more changes there,
it will be not quick fix, maybe another 2 year ;)

Just in time for 4.0 final then ;-)

Fixed with #34808

OK Just realised the comment about keeping it open for J4 ?

The fix for Joomla 4, please test #36345