Thanks for posting your question in the Microsoft Q&A forum.
According to your problem description, I found the same case in other Microsoft forums, have you tried any of the solutions.https://answers.microsoft.com/en-us/outlook_com/forum/all/shared-calendar-not-syncing/93a40501-e81a-4210-9581-0a5161964f39
If my answer is helpful to you, please mark it as the answer so that other users can refer to it. Thank you for your support and understanding.