why does the deleted mail folder empty all deletes up to the last 7 days when the folder is opened?

It is near impossible to recover a deleted email more than 7 days old because the deleted folder empties all older deletes immediately on opening

Can you check what setting you have on your server? eM Client do this when email server deletes them from that folder (outlook.com for example has 30days and then it deletes them for good)

also can you check if you have turned on automatic archivation in Tools - Settings - mail - Automatic archivation.

with regards