The characters at the end of the subject line change with each notification. However, only the 4th character from the right changes. I don't know where these characters are coming from. I deleted the subject line, then saved and published the workflow. The characters still showed up in the subject line for the workflow notification. I deleted the flexi-task and rebuilt it, and the characters still show at the end of the subject line.
Example Subject Line: Response required - HR Consultant Approval Ad-Interim Pay Request Form [#EEEROHHTOHHNUVG#]