Mosin was Russian. (MO-seen)
Nagant was Belgian. (Na-gau)
The Russians never called the rifle the Mosin-Nagant.
Although Nagant sued, and was paid hush money, the fact remained that no appreciable part of his design was incorporated in the final rifle, and that Mosin’s design won the competition. As that wasn’t well known at the time, the Western media just called it the Mosin-Nagant.