Taking the steps to check your mailbox 📫 behaviors was a good one and your right in the advanced settings there is no option for deleting after 7 days. In Settings > Mail > Gmail> Account >Advanced > Deleted Mailbox you simply have the choice to select if you want the trash to be deleted only “On My Phone” or “On The Server”. The destination that you choose is where the iPhone should delete from.when you went to the mail advanced settings was there a Blue ✔️ beside the trash for the iPhone or on the Server? For IMAP accounts (such as iCloud or Gmail), copies of messages are stored on incoming mail servers until they are deleted. I am sure you have already checked but definitely check the Trash folder in all devices in the Apple Mail App and Web Mail. If they have deleted from the server as well as the Mail App and this happened in all devices I would attempt to contact the ESP and see if they can provide any help however if they are unable to help Call Apple Care 1-800-275-2273. There’s still a lot of information needed to help determine what may have caused the issue and the best way to resolve it. There’s some articles that might be useful attached below.
iOS Articles
https://support.apple.com/en-us/HT208661
https://support.apple.com/guide/iphone/delete-and-recover-emails-iphb02be90ba/ios
https://support.apple.com/en-us/HT207428
https://support.apple.com/en-us/HT201232
Good Luck 🍀
