This is something that I’ve noticed for years. If you delete the end of a tag then edit something in else in the tag the deleted portion returns. Roe example, if you have a title string such as “Jump To It (Original Extended Version)” and delete the portion “(Original Extended Version)” then continue to edit the remaining string “Jump To It”, without saving, to “Jump to It” the deleted portion returns giving you the string “Jump to It (Original Extended Version)”. This occurs in all the fields.
I think you have become a victim of the auto-completion.
Please note that the automatically added part gets selected and you can easily overwrite it. As soon as you enter something that is not part of the original string, the auto-completion will stop.
The auto-completion is case-insensitive.