Re: Instead, I received an email in my personal Google email account saying that a calendar entry had been added to my Googe calendar
Never heard of such emails -- was it an invite (which would be wrong too, but it's just to help me understand)?
Just tested your scenario with a different calendar provider, this one:
https://play.google.com/store/apps/details?id=org.sufficientlysecure.localcalendarWorked as expected, the event was saved and no mysterious email.
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Can you try this "local calendar" app to see if there is a difference?
Oh and before saving the event in AquaMail, did you select the Deja calendar in the drop-down?
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And one other thought:
If this was in a Gmail account -- Gmail has a setting to automatically save all incoming invites in your calendar, and IIRC it's even enabled by default. Done by the cloud, not the Gmail app.