Mail does not hide
I have a 2017 iMac running Big Sur. When Mail is set as a login item and the 'Hide' box is ticked, it never the less launches at start up. This has been true for years, including on the iMac that is the current machine's predecessor, and on all of Big Sur's recent predecessor OS.
What is the definitive answer to restoring that - hide mail at log in - function?
It is not a power setting issue. It should not require a command line or a workflow. It should be within the remit of Apple's engineers to fix this long standing OS weakness.