I think I’ve just ran into a bug (with Ghost 4.2): I sent two newsletters this week, and for both of them the number of opens is vastly inconsistent between Ghost and Mailgun. The number of opens is not zero (so this is not an API key issue as explained here: How to troubleshoot email open rate being 0%?), but the difference is huge (Ghost displays an open rate of 1-5% when the actual open rate is something around 50%).
It seems that Ghost starts to properly count the number of opens right after the newsletter has been sent, but then suddenly stops.
Any idea what could be the source of the problem?