Table headers no longer sticky after merging cells

Summary

When working with a table with sticky headers, merging cells in the header row breaks the sticky header functionality.

Environment

Confluence 5.10.7

Steps to Reproduce

1. Create a table with a header row
2. Merge two cells in the header row
3. Save the page
4. Test whether the headers stay in place when scrolling down the table

Expected Results

The merged cells/columns would make no difference to the stickiness of the header row

Actual Results

Before merging cells:

Scrolling after merging cells - headers scroll too:

Workaround

There is no workaround.