Nothing-at-Stake is a theoretical problem invented by Vitalik so that he can promote his Casper PoS mechanism. It’s a far-fetched problem without much connection to reality. It has never been observed in practice in NXT and Ardor, we haven’t even seen the software which is supposed to be used by the forgers (the block generators, miners) to build on more than one chain. Chances are such software is not even developed, let alone adopted by a significant part of the stake holders. Why forgers don’t forge on more than one chain? The inventor of the theory should answer that, but probably because there is no incentive to do so, considering the possible losses.