On iOS, Proton Calendar is treated like any other third-party app by the operating system. When the app hasn't been opened for a period of time, iOS puts it into a background state that prevents it from sending notifications for scheduled calendar events. This causes users to miss important reminders without any indication that something is wrong.

Implement a "wake-up" reminder notification similar to what Trakt.tv does for their app. If Proton Calendar detects it hasn't been opened in a while, send a push notification prompting users to open the app to continue receiving calendar notifications reliably.