You are assuming that the federal government has the authority to alter the borders of the states, it does not:
Art 4, Section. 3.So then, a treaty made not under the authority of the United States is not [nor can it be] the supreme law of the land.
New States may be admitted by the Congress into this Union; but no new State shall be formed or erected within the Jurisdiction of any other State; nor any State be formed by the Junction of two or more States, or Parts of States, without the Consent of the Legislatures of the States concerned as well as of the Congress.
The Congress shall have Power to dispose of and make all needful Rules and Regulations respecting the Territory or other Property belonging to the United States; and nothing in this Constitution shall be so construed as to Prejudice any Claims of the United States, or of any particular State.
Treaties that violate the Constitution are invalid
As I said, I’m not familiar with the specifics of this case.
But the borders of WY are exactly as they were. As I understand it, the EPA has ruled that certain territory within those boundaries is within, rather than outside, an Indian reservation. I believe this is an issue of sovereignty, not private property rights.
Reservation boundaries have been changed in the past, as a result of lawsuits and otherwise, and no doubt will also be changed in the future. I have no idea whether this particular decision was made properly.
Your Constitution quote isn’t even relevant. Nobody has tried to create a new state or split up an old one.