(Speculation) Anakin didn’t learn to become a force-ghost the same way that Yoda and Ben did, he only turned into one because he was conceived of the force.
(Canon) Qui-Gon didn’t complete his training with the Force Priestesses so he was unable to manifest physically, only as a voice.
I suppose my argument is that they both turn into force-ghosts, but not the same way that Yoda and Ben did. Yoda and Ben’s method required the body to “disappear”.