There is a scene in ROTS with Yoda and Obi on the Tantive, which explains this very thing. Yoda, Kenobi, Qui-Gon and Anakin are the only Jedi so far in to come back in ghost form, not all of them.
Well since two of those four disappeared and the other two didn’t, I fail to see how a scene in ROTS explains anything.
Qui-Gon is the first guy to figure it out though.
He “learned the path to immortality, and returned from the netherworld of the force”. He learned to manifest sometime after death, not before. So no disappearing.As for Vader in ROTJ they either goofed, or Anakin had to learn from the rest of them after he died, since he likely never learned it in life, unlike Yoda and Obi-Wan who learned it from Qui-Gon.
Palpatine stole the technique from his master & promised to teach Anakin in ROTS,…to save Padme…Luring Anakin to the Dark Side (daft I know)…Palps continued to teach Vader,…but unfortunately a Sith can’t live on through the Force when he dies,…only a Jedi…(a bit like Heaven…sinners are bad),…but Vader redeemed himself at the end,…Luke incinerated his body & the spirit of Anakin lives on with the Force (daft I know).
Qui Gon died & was cremated at the end of TPM,…George totally forgot about this plot hole so he crammed in that Qui-Gon has been studying this technique but had not completely mastered it,…but after his body was torched,…he lived on through the Force…& came back as a spirit to Yoda & told him the special trick,…so Yoda told Obi-Wan to give Qui Gon’s spirit a shout while he was being a hermit on Tattooine so that he could to the live on through the Force thing…(daft I know)
J
So what you’re saying is that George forgot but he didn’t forget?