Unable to remove photos from Portrait Media in Photos on iMac

Sorry, it is a feature, It is an automatic album that Photos creates that uses some secret criteria to determine which images are portraits. It does a similar thing with panoramas.

All you can do is ignore it.

As Keith Barkley implies, you have to read the name of that "album" as "Things we think might be Portraits." It can be handy, but not as handy as it sounds. Pretty much the same applies to "Things we think might be Screenshots," "Things we think might be Panoramas," and "Things we think might be Selfies." Probably others, too.

In other places, you can find "Things we think might be Trips," etc.

The shared albums are meant to be helpful by structuring our library automatically according common criteria. I am using them mainly as a starting point, then creating standard albums from them, where I can freely remove the items that I do not want.

On macOS Golden Gate the Mac OS collecting all photos with a depth map in the "Portrait" collection, even photos with no people in them. But my iPhone is now automatically adding a depth map to most landscape photos, not just the to the photos where I have enabled the portrait mode.

Previously I have been able to remove the depth map by turning off Portrait, then exporting the photo. The exported version would not have a depth map any longer, but since Golden Gate the depth map seems to be exported with the edited version. when I reimport the exported photo it will still show as a portrait. I have not yet found any way to remove the depth map from photos where I do not need the depth map.

Note, that like all Albums, having images there does not remove them from anywhere else in the Library. It is basically just a pointer back to the thumbnail in the main Library. So while they may be annoying, they don't use any appreciable disk space or prevent you from using your images any where else in the Library.