I would like to first confirm with you:
- According to the information, are you using Exchange Online (Microsoft 365)?
- Is firstname.lastname@example.org a shared mailbox or a normal user mailbox? To me it seems to be a normal user mailbox which shares the calendar with others.
- What is the detailed permission setting of the calendar folder? For example, did you assign Owner or Editor?
I tested in my lab and found that if someone sends response to the newly created meeting, the "last modified by ..." part should be changed to this user.
Would it be possible that Boss sends response to these meetings after they are created?
If not, does the Organizer of the meeting show the correct user?
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