A cloud-based service included in Microsoft 365, delivering scalable messaging and collaboration features with simplified management and automatic updates.
From your description this issue may be caused by retention policies.
Do you have any retention policies assigned to your mailbox which may work on the calendar folder?
If you are not sure about that, please contact your Exchange admin for help.
Besides, have you configured this mailbox on other devices like mobile?
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment". Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.