Have you had a conversation with this contact before Touting them an email?
We rely on subject lines to track replies, our system searches your inbox and when it finds a match it marks it as “replied”. Google refreshes your token every hour, so the authentication has to be refreshed every hour.
Typically we can maintain this seamlessly, but every now and then when it re-authenticates it will re-scan your inbox to detect a response.
If you sent an email to your contact in the same thread, subject lines will match and it will trigger as a reply.