Events can be created in Proton Calendar manually or can be automatically imported when attached to emails.
For example, when booking a meeting time in a third party calendar, it's common for an email with a calendar invitation to be sent.

This feature request relates to email attached invitation handling.

If the event is accepted then notifications are sent. Expected
If the event is declined then notifications are not sent. Expected.

If the event has been neither accepted or declined then notifications (e.g: 15m & 60m before) are not sent.
This was unexpected behaviour for me coming from Google Workspace/G Suite/? and other systems before that and has now caught me out twice.

When looking at the calendar on iPhone or browser mail sidebar, pending events are not necessarily easy to distinguish from accepted events.

iPhone: both pending and accepted have a green circle. Accepted have a solid circle, pending have grey lines inside but at a glance it's green.

Browser: both pending and accepted have a green left bar and dark/light main bar.

Whilst it's best to reply to invitations, I believe notifications should still be sent regardless, or at least make it optional (defaulted on IMHO).

Additionally, changing the pending invitation theming would help, perhaps by using a suitable contrasting colour instead of a green variation.