The Republic of Texas specifically negotiated the right to split into as many as five separate states in the treaty joining the U.S. Too bad they didn’t think to negotiate a right to secede.
It probably never came up since before the Civil War it was always a presumed right of the States. Something about the consent of the governed in all of the original documents.
The Republic of Texas had been fighting for 10 years to join the Union, ironically enough opposed by Congressman Abe Lincoln.
It surely didn’t occur to them that they would be fighting to leave not long after.