The fast blinking light might indeed be because it's not an official Line 6 battery, and the Variax might have some sort of built-in check for that.
But here's the thing, if it's showing 4 dots and holding up during your gigs, it sounds like it's performing just as well as the original. Plus, you mentioned it has a slightly higher capacity, which could even be a bonus.
Sometimes, these third-party options can be hidden gems. Just keep an eye on it, especially during extended use, to make sure it continues to perform as expected. If it does, you might have found yourself a cost-effective alternative without compromising on quality. You can always check out options like ACE Battery storage solutions for more alternatives.