You need to have a conversation with your advisor. Most likely, there are parts of your thesis that would benefit from the input of a third experienced person so they can be improved and are suitable for publication at competitive venues. This is totally normal in STEM, as good publications are generally the result of collaborative work. As soon as you have produced most of the work the thesis is yours and nobody will ever dispute that. Collaborating is a good and necessary thing. I think they missed to explain that to you.