Accepted event invitation did not populate Lightning calendar

I have the Lighting calendar add-on in my Thunderbird portable. In the past, when I received an email that was an invitation to an event, I would click the Accept button, and the event would populate the appropriate day on my Lightning calendar. However, this past week, when I accepted an event, instead of populating Lightning, it populate the calendar provided by my email service, Outlook on the web (specifically, Office 365 Business Essentials). While I'm glad it's at least populating some calendar, I would much prefer it to continue behaving as it has in the past and populating Lightning in Thunderbird.

I can't find any settings in Options that appear to control the behavior. Any assistance would be welcome.