"Delete source branch when merge request is accepted." option is not working intermittently

When I create a Merge Request ticking the box "Delete source branch when merge request is accepted.", the branch doesn't get deleted. I can see the message "The source branch is being deleted".(Attached Screenshot)

Version : Community Edition (CE) - 13.6.7

Steps to reproduce

Create a feature branch and open a merge request by selecting "Delete source branch when merge request is accepted."

What is the current bug behavior?

The branch won't get deleted.

What is the expected correct behavior?

The branch should be deleted.

Please advise ASAP. Thanks in advance.