I encountered this issue after i deleted everything in my current project and tried to import using “add to scene”. after the file imports, it creates a new projects and also adds the deleted objects. At first I thought I might have pressed the wrong button or maybe i pressed the wrong “add to scene”, but after doing it again, I was able to replicate this same behavior.
I just tried it with a new project + default sphere, deleted the sphere and added a sculpt to the scene via add to scene - no deleted objects (the sphere). Unable to replicate the behaviour my end.