Time travel violates the law of conservation of energy. By travelling back in time, you yourself are injecting more mass/energy into a system. So it's great as a science fiction trope, but if you're going to talk physics, that needs to be addressed.
Take this with a grain of salt, as I'm vastly underqualified to even form an opinion on this subject.
We know that all of the laws of physics we observe at the local scale are time invariant, but that is not necessarily true for any law we haven't discovered yet. While on the local scale that feels so unlikely as for it to be basically considered "inviolable without extraordinary evidence", that is not necessarily the case in the community for laws at the cosmic scale.
So isn’t the simple layman’s answer that the “system” also spans the 4th dimension and you are just displacing mass around in a 4D space instead of the typical 3D space?
when you move to a different timeline, you are moving mass from a timeline to another timeline, so total mass in multi verse remains the same