This was not the first time. Esra, in Rebels, when in a temple meet Yoda's ghost without having any connection.

Regarding the 'weak' ghost appearance from Qui Gon, is also explained before in clone wars series. When Qui Gon 'appeared' to Yoda they explained that he could not finish the train to become a 'full ghost' and then the train for that is showed in a couple of episodes.