The expansion of the balloon changed the weight of your sealed flask because you and your entire experiment are submerged in a fluid: air.

Just like water, air is a fluid, and fluids buoy up objects. The upward buoyant force on any submerged object is equal to the weight of the fluid displaced by that object—this is known as Archimedes’ principle. By increasing the volume of your sealed flask, you cause it to displace more air, increasing the buoyant force on it and reducing its weight. Here's the thing to remember: Scales measure weight, not mass. The mass stayed the same due to the law of conservation of mass, but because of buoyancy, the weight went down!