is it in the task when you click New task? If so, its either in the form - are you using a custom form? - or a macro doing it.
No, I don't think it is ever there when a new task is first created. It appears later when the task comes due. I'm not sure if it happens when the task first comes due, after a snooze, or after a recurrence. And it doesn't happen with every task.
I did a little testing and I think I discovered when it happens. It's right after a task with a recurrence comes due and is marked complete so that that instance of the task is completed and a new instance is created. The new instance gets the extraneous text.
I captured 6 screen shots illustrating the process. The forum software wouldn't let me upload all 6, so I created a PDF with some explanation. It wouldn't let me upload that because it was too big. (sigh) So I created a DropBox folder and put it there. Here's the link if you want to look at it:
https://www.dropbox.com/sh/i5855t31rrs1tyo/1pyqKpyeQg
I don't believe I have any custom forms or macros. At least I'm pretty sure that I didn't intentionally create any.
This has been happening for many years...