I have a model made up of 10 separate objects. I had all hidden while working on a new object. When I alt/h to unhide, 5 of the objects remain hidden. They are still listed in the collection, and when selected, geometry of the object is listed so they are still there.
If they are all in the same collection and the ‘eye’ icon is active in the outliner, I wonder are you in “local view” mode. Try pressing the ‘/’ key on the numpad to see if they magically reappear. Local mode isn’t limited to one object, but rather the objects you have selected, so it could be you’ve hidden them then dropped into local view.