It could be due to a browser update that now treats these cookies differently or cleaned them up as part of the update, or is now seen as a different browser by the forum software. It could be due to browsing in a private window. It could be due to a change in the validity of the cookie. It could be due to a different bit of software cleaning up stuff that was seen as unnecessary. There are more reasons. But unless things start acting completely differently from here on, I don't think there is any reason to be suspicious. It's part of online life.