I realize that the US has violated many treaties with native peoples in the past.
Are there any active treaties with Native Americans that are currently being violated?
Plenty, if you ask the Native Americans themselves. (Suffice it to say, the U.S. doesn't really take the stance "we are currently breaking the law.")
Take for example the Fort Laramie treaty of 1868. It created the Great Sioux Reservation in the western Dakotas, only for the U.S. to blatantly violate the treaty once it became clear there was gold in the Black Hills. After a period of warfare, the U.S. forced Sioux leadership to sign a second treaty at gunpoint.
The problem, as the Supreme Court noted over a hundred years later, is the Fort Laramie treaty's language specifically forbade this sort of landgrab and laid out provisions for how the Sioux could cede the land, provisions that clearly weren't met:
Under the Fort Laramie Treaty of 1868, the United States pledged that the Great Sioux Reservation, including the Black Hills, would be "set apart for the absolute and undisturbed use and occupation" of the Sioux Nation (Sioux), and that no treaty for the cession of any part of the reservation would be valid as against the Sioux unless executed and signed by at least three-fourths of the adult male Sioux population. The treaty also reserved the Sioux' right to hunt in certain unceded territories. Subsequently, in 1876, an "agreement" presented to the Sioux by a special Commission but signed by only 10% of the adult male Sioux population, provided that the Sioux would relinquish their rights to the Black Hills and to hunt in the unceded territories, in exchange for subsistence rations for as long as they would be needed. In 1877, Congress passed an Act (1877 Act) implementing this "agreement" and thus, in effect, abrogated the Fort Laramie Treaty.
For any trained lawyer, this would not be a particularly difficult case in pretty much any other circumstances. The second "treaty" did not abide by the legal requirements set out for parcel of land A to be transferred from Party B to Party C, therefore, the land belongs to Party B and the treaty is still in effect. And to the tribes, this is the end of the matter: they own the land, give it back.
But the United States' view is that the illegal seizure must be answered with compensation. And so about a billion dollars of compensation is sitting in a bank account that the Sioux can access at any time. But the tribes haven't touched a dime, and continue to fight for the actual land.