I have a hidden photos album on my iPad, iPhone and Mac. However, on my Mac, the photos remain visible on the “recents” album. How can I hide them?

I'm not aware of the ability to hide albums - it is only possible to hide images, in which case they will disappear from everywhere (including recents), EXCEPT the hidden album.

However I am on BigSur (photos 6). Are you still on 10.11 as stated? If so the behaviour might be different, but I can't check.

The behaviour of hidden photos has changed between system versions. In " OS X 10.11", as shown below your question, hiding has been only meant as a way to hide photos from the Moments, to make the Moments more manageable. It has not been intended as a way to keep photos secret. Hidden photos have still been visible in all albums. This has changed in the more recent System versions. I am not sure, when it has changed, but at least since macOS 10.14 Mojave a hidden photos will be hidden everywhere and only visible in the Hidden album.