https://wordpress.org/support/topic/input-sanitization/

For example, say you are editing Auto Terms->Edit Auto Terms->Stop Words.

If one of the stop words contains an apostrophe ,the you naturally sanitize it by escaping it with a backslash. However, this occurs each and every time the input is saved, so you can end up with a string of backslashes thus:
I'm becomes I'm and on subsequent saves, another backslash is added