Hi @Leo B ,
Yes, this is an expected behavior by default, To change this, open Settings > Calendar > Events and invitations (or directly https://outlook.office.com/mail/options/calendar/eventAndInvitations) and toggle the Delete invitations and responses that have been updated setting.
Please refer to this similar post for more understanding: https://learn.microsoft.com/en-us/answers/questions/970387/accepted-invitation-emails-are-moved-to-39deleted.html
Hope this helps.
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