Actually had the same problem tiling a bathroom last year. The floor dipped toward the drain and the laser line looked perfect. Ended up checking for slope with a long straightedge and shims before setting anything. Now I always double-check the high and low points of the floor with a level itself, not just the laser guide. That laser is just a tool, you still gotta know what the surface is doing under it.
How can you be sure the laser messed things up? Lasers give a straight line on any surface. If the floor is sloped, that's the real problem. Maybe the tile was laid without checking for the slope. I've seen jobs where installers blame the tool but it's their setup that failed. The laser just shows the truth.