It's difficult to say anything about this, especially without knowing which manga you believe you were following and then later weren't (although even knowing that might not help). Obviously data doesn't just magically disappear from the database, so if this unfollowing really happened something must have triggered it, but as far as we're aware the only way for that to happen without the user's knowledge is some kind of a mistake on their part.
The point is, if this is actually a bug on the site, we have no idea how or why it would be happening, especially without some kind of a clue to start with.