Some slight internal movement can be normal on a Canon 50mm f/1.8, especially since this lens is known for relatively basic, lightweight construction. One user with the same lens reports feeling movement when gently shaking it, likely from the focusing mechanism, while still having a lens that functions properly.

That said, no one can confirm whether your particular lens is defective without inspecting it. A small amount of play does not automatically mean there is a problem.

The best test is performance: if autofocus works normally and the images look as expected for this lens, it is probably fine. If image quality is clearly much worse than typical reviews suggest, or if the movement seems excessive, returning or exchanging the lens would be reasonable.