In older system versions we could see the the hidden photos in the albums - hiding has originally been intended to be able to hide photos from the library temporarily, without having to delete them, to make it easier to manage the library views. Hiding did only apply to the Library view, not our own albums. But many users requested a hiding tool to keep confidential photos hidden everywhere, also in the albums. So Apple changed the behavior of hte hidden albums, with macOS 10.14 and iOS 13, many years ago. Since then we are no longer having a tool to take photos out of sight just in the Library view, but a tool to hide certain items completely. In the most recent system version we can even protect the Hidden album with Face ID.
On your Mac mini you can make the hidden photos visible in smart albums, but not on the iPad and iPhone, because the mobile devices do not support smart albums.
To see the hidden photo in an album without having to unhide them use a smart album like this on your Mac:
- File > New Smart album
- In the smart rules dialog for the smart album set the switch "Match " to "Match All".
- Set the rule to "Album is" and select an album with hidden photos.
- Add a second rule "Photo is Hidden".
This new album will show you all hidden photos in the album you selected. But you may need to unlock this album with Face ID if you have Face ID enabled for the Hidden album.
Any smart album with a rule "Photo is Hidden" can show you the matching hidden photos.