Confluence tables format breaks when header merged with rows.

Problem

When we publish the page which contains table, the table formatting breaks. 

Environment

Confluence 7.10.x

Steps to Reproduce

Step-1. Create a table on a page (ex 3*3).
Step-2. Fill cells with some data. As it shown in attached "image-2021-02-04-11-48-06-963.png" and save the page. Everything looks fine so far   

Step-3. Edit the page again and merge the Heading1 with other rows 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.  

Notes