Deleting first field group from the top doesn't work

Steps:

1. Add custom block to configuration using nested attributes.
2. Create Landing page.
3. Add this block.
4. Open block settings.
5. Add a few new instances of the field group.
6. Delete the first field group from the top.
7. Submit.
8. Open block settings again.

Actual result: Field group is not removed.
Expected result: Field group is removed despite the order.