When tracking events with multiple Proton Calendar calendars, then events on these secondary calendars end up triggering spurious iOS notifications. The notifications appear from Proton Calendar when using Proton Calendar notifications, or Apple Calendar when using Apple Calendar notifications. In both cases, I have the iOS client application set to import only a single, main Proton Calendar calendar. But the extra ones keep slipping through.

The secondary calendar events do not appear on iOS Proton Calendar nor Apple Calendar, just the notifications.

As a workaround, I have to remember not to use extra Proton calendars. This makes it harder to manage complex collections of events, including events that happen to be optional or purely informational.

One more bug in a growing list of problems with the Proton Calendar service.