Copy/paste table cells does not work as expected when header cells are merged

Issue Summary

Copy/paste table cells does not work as expected when header cells are merged, enclosed is a video demonstrating the behaviour.

Steps to Reproduce

1. Add a table to a page (or use the storage format from this ticket)
2. Merge first and second header cells
3. Try to copy/paste multiple cells
4. Content is not pasted

Expected Results

User should be able to copy/paste cells within a table even when the header cells are merged

Actual Results

User is not able to copy/paste cells within a table when the header cells are merged

Workaround

As a workaround, you can split the merged header cells by clicking on the split icon from the editor toolbar