Confluence table format breaks when right column is merged having a left heading column

Issue Summary

The table formatting breaks on Confluence Data Center and Server when right column is merged having a left Heading column.

The error only appears when using a left Heading column with right column being merged.
The error does not appear when we use no Heading, a Heading row, or a Heading column and a Heading row simultaneously.

There was also a similar bug: Confluence tables format breaks when header merged with rows. in the past which was fixed. However the steps to reproduce this bug are different.

This is reproducible on Data Center: Yes

Steps to Reproduce

Step-1. Create a table on a page (ex 3*3) with a Heading column.


Step-2. Fill cells with some data and save the page. Everything looks fine so far.

Step-3. Edit the page again and merge the last column on the right, using merge cells button.

Step-4. Click to button save and it will lose the table formatting as below: 

Expected Results

It should not lose the formatting of table and show 3*3 table as follows:

Actual Results

Table formatting breaks. 

Workaround

1.  Install Confluence Source Editor plugin. 
2. After installing, please navigate to the problematic page. 
3. Edit the page and click the source code <> in the right top.
4. search for <tbody> and </tbody> tags and replace with <thead> and </thead> tags.
5. Click on Apply and save the page.