I haven’t experienced this myself, but my notifications are handled on a separate notification server for testing, so it may be something that only affected the production server. I just restarted both servers because they have been running for nearly a year now without a reboot.
I should have some time this week to look into this further and see if I can’t find what may have been causing this.
Additionally, Arctic does not clear notifications when they are marked as read in the app. I just fixed this yesterday, so the next release will properly clear iOS notifications when they are read in the app.