I haven't found case bulges to impact accuracy, however, there are a few rounds I reload for that will do it.
Some new brass is much smaller than the diameter of the bullet. I ran into this with some "JLT" .357 brass I bought a few years ago. I'll never buy that brand again. On the initial loading the bullet bulges the case significantly, because the brass is out of spec. It's fine once it's been fired, therefore expanded, and sized again. The diameter of those cases from the factory are not correct.
Does the bullet seat normally after the initial loading? If so, then your new brass is out of spec.
Some dies also size brass down a little smaller than others. I sometimes get a bulge with 9mm Remington brass when loading with my Lee dies. It doesn't impact function and accuracy is great, so I consider it a non-issue for me.
Remington brass is considered thicker in 9mm, I think, which could also be contributing to the issue.
Have you measured the internal diameter of the case compared to the base of the bullet?
Belling helps the bullet start in the case, however, if the case is too narrow for the bullet, like my JLT brass, then it will bulge the case as it's seated.
The other issue could be bullets not seating squarely in the case. You should be able to look at a finished round and tell if that's the case or not.
That can be caused by misalignment between the shellholder and the die or possible the bench flexing as you seat the bullet. If that's the case, then the bulge will only be on one side of the case.