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Below is how it looks in my editor. The values of first field group that I create are not saved. This happens on every post, both when creating and updating. After the post is saved, the first field group always appears empty like in the screenshot below.

A few other things I noticed:

I cannot remove the empty field group. If I delete it and save the post, it reappears after I refresh the editor.
I can reorder the field groups, but after refreshing the editor, the empty one will reappear at the top.
The empty field group has the .rwmb-clone-template class. The markup looks like this:

<div class="rwmb-input ui-sortable" data-clone-empty-start="0">
  <div class="rwmb-clone rwmb-group-clone rwmb-sort-clone rwmb-clone-template">
    <!-- empty field group -->
  </div>
  <div class="rwmb-clone rwmb-group-clone rwmb-sort-clone">
    <!-- next field group -->
  </div>
</div>