Didn't Google stop scanning Gmail for ads targeting?
https://www.nytimes.com/2017/06/23/technology/gmail-ads.html
https://www.nytimes.com/2017/06/23/technology/gmail-ads.html
US law interpretation of IMAP. Only with a POP3 service you are safe.
Retrieving email via the POP3 protocol typically deletes the email upon retrieval, making it impossible for the third party to comply with requests for already-retrieved emails.
What POP3 client are you using that does this? I've not seen it before. It gets marked as read but that's it.
It used to delete from the server after downloading locally. This was when most email mailboxes had a capacity of 10MB.