Possibly hacked?
If i change my setting to messages erased after 30 days but it is being changed back to FOREVER by another device added, this does not mean my phone could be hacked by someone elses phone
iPhone XS, iOS 14
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Check your device list to see if there is a device signed in that you don't recognize. You can remove the device using the steps in the article below. As a precaution, change your password.
Apple ID device list -Check to see where you‘re signed in.
What to do after you change your Apple ID or password - Apple Support I suggest doing this before you change your Apple ID or password.