It looks like it could also be an air pocket. If you poke a hole on the very top of your cake using a wooden skewer, it can act as a chimney and allow trapped air to escape rather than forming a bulge. Hope this helps!
this is very interesting, if it were an air bubble you would probably see that once it was cut, have you tried checking it out once it gets cut? Maybe that will give you a better idea of what is going on.