When accepting an event invite, Lightning tries to guess the calendar to add it to based on the email it was sent to. I think this is implemented here:
Unfortunately, the invites I receive are often sent to the wrong email, and lightning creates the even in the wrong calendar without offering an alternative. I have two calendars (a professional and a personal one), and I receive events for both on various email addresses, including those attached to these calendars. (For example, I sometimes receive meeting invites sent to my personal email for professional events, and vice-versa).
Could we design a UI that allows users to pick the right calendar manually?
Here’s a relevant thread from 2016: http://forums.mozillazine.org/viewtopic.php?f=46&t=3003695