If an outlook-user invites another outlook-user to a meeting outlook will automatically import the meeting as pending in the corresponding calendar. This way colleagues can see that one might be in a meeting, and it's possible to accept the meetings from the calendar-view instead of through the inbox.

When an invite from Proton Calendar is sent this doesn't happen, and you have to accept invitations through the inbox.
Since Outlook behaves differently i assume it's something with the formatting in Proton Calendar - and this would be nice to have fixed.