Is it normal to have a bulge along the seam between pieces of drywall?
A typical horiontal seam for drywall occurs at 4ft.  Usually its the best seam because the 2 long edges (of a 4'x8' sheet) are usually tapered, making a blended seam nearly perfectly flat.

Fixing this depends on how perfect you want it to be.

Nearly perfect: rough up paint 15" above and below bulge.
Using bulge as a guide, put down general purpose joint compound above and below with a 12" taping knife.  You are building a ramped area that will fool the eye, because it extends over 24"

When dry, lightly sand with fine grit sandpaper blocks. Shine light along wall from above. If no prominant shadows emerge, paint and enjoy.

If not, recoat and resand. I've never had to go beyond +/- 12" to hide a seam.

Is the wall flat? No. Is it not noticeable by casual inspection? Probably.

Perfect is a whole 'nother chapter.