This might be a Bug. When I go to Create a 2D sketch on a plane it un-hides other objects in one of my Folders. I think the objects that are in the folder the new sketch plane is will be placed are the ones being displayed. But I am not sure about that. Also How do I specify which folder a newly created sketch plan is going to live?
I think this happens when your new sketch is on the same plane as a hidden one. I’ve found that creating a construction plane and moving it slightly off that plane will help.