Declined event keeps popping up on Calendar?

I received a calendar invitation that started in 2020 with no end date. I have declined this event multiple times on my Exchange email and on my iPhone. However, it keeps showing up, and now when I select "Decline for All Events" it wouldn't take, and I would have to go into each day (mind you this is a recurring event every day since 2020 with no end date!) and decline one by one. I don't know what else I can do to stop this from popping up on my calendar. I've tried to delete and re-add the email with the calendar, turn calendar off and back on, etc. When I go to my Outlook, the invitation doesn't show up on my calendar, so I'm thinking this might be a sync issue with iPhone? It just seems really annoying and makes it hard for me to do scheduling because it always lets me know that there's a contradiction when I am actually trying to schedule an event. If any one has run into this issue and have a solution, or any suggestion on how to fix, I would gladly appreciate it. TIA!