The retention "age" for items in the Deleted items folder is calculated differently, as detailed here: https://learn.microsoft.com/en-us/exchange/security-and-compliance/messaging-records-management/retention-age#determining-the-age-of-different-types-of-items
Other than that, there are some caveats for the tag itself, as at one point Microsoft decided items within the Deleted items folder should not be automatically removed. Long story short, they ignore the "default" Delete items tag, so you either have to create a new one, or create a new MRM policy (or rename the default one). Details are here: https://www.michev.info/blog/post/5868/make-sure-deleted-items-are-automatically-removed-from-microsoft-365-mailboxes