I have the same issue and below are the details from my side on this issue.
Meetings are scheduling directly from the Outlook app for Windows.
We are using "Send Response now" option while accepting the meeting invite and the organizer is receiving the response email also the organizer can see the response as "Accepted" in tracking and I being an Attendee can see it as "None".
Automatically process meeting requests and responses to meeting requests and polls" is checked.
This mailbox is the Primary Mailbox not a shared one.