I'm a fan of GHS Boomers. They're one of my go-tos for nickel rounds. They're stiff, but they break in nicely. To my ears, they're warmer than many other rounds.
so, it's a problem of twisted strings. try the method I told you: detune until you can "move" the strings, and pull them out of the bridge, and then release them, like a rubber band. probably they twisted during the mounting process.