I noticed this on the app and thought it was just a bug there, but it's showing up on desktop, too. It looks like at some point the indication of whether you've read a comic episode or not went back to unread? Like, for series that I'm currently reading the older episodes are showing up as unread again.
It's not really a big deal, but it is annoying that I won't be able to tell if I've read an older series or not.