I have two messages from customers received within the past 24 hours that I have answered twice within the time limit now but as soon as I send a reply, they are marked as "First message from potential buyer" again. This glitch could cost someone their star seller status. Please investigate this bug.
This is a glitch which goes away after the next day or so. It happened to me as well. It will say Good Job for answering so quickly, but the blue text re: first time a Buyer is reaching out still stays. It does disappear the next day. Just make sure you look at your Customer Service (Star Seller Stats), so there isn't one that got slipped in there, something other than the glitch.
@Kultur: Based on the numerous threads about this, the new message banner not going away appears to be only a user interface glitch is not affecting any stats such as Star Seller. As long as you see your reply on the thread you should be okay.
There have, however, been reports about other Message issues that are impacting a shop negatively, particularly Messages replied to using the app.
Best to check the applicable CSV file on your Star Seller page regularly. Keep in mind that usually a new incoming thread is not included until after the 24 hour reply window has passed regardless of when you replied and that Star Seller stats (and the CSV file) seem to be updated only once per day so there may be an additional day's delay.
Thank you for the replies and putting my mind at ease about this. : )