Calendar app on desktop shows declined Outlook invitations

I use the calendar app on my desktop and phone. I have multiple accounts that feed into my calendar, including gmail and outlook addresses. Sometimes I receive event invitations in my outlook account that get added to my calendar even if I decline them. If I delete them, nothing happens. They only show on the calendar on my desktop, not on my phone. They do not appear in the outlook calendar themselves. I can delete the outlook calendar and re-add, which solves the problem for a period of time, but inevitably, an outlook calendar invitation retriggers the problem at some point.