Objects are decimating down to a single face? Is this normal?

Today when repeatedly decimating, objects have been decimating down to a single face, or sometimes a couple of disconnected faces. I don’t remember this ever happening before. Previously I’m sure there would come a point where the object would just stop decimating - it wouldn’t end up at a single face. Am I misremembering, or might I have changed a setting without realising, or is this the expected behaviour? Could this be a bug? Any help appreciated.