The additional vocabulary being added for kanji with no vocab appears to be scheduled in exactly level order. Like, if you take the kanji missing vocabulary and order them by level, that pretty much exactly matches the order in which new vocab is added in the release schedule. See:
They’ve gotten up through level 54 only. 瑠 and 璃 are 57 and 58. I’m guessing the next batch of updates should cover all the remaining kanji, and we’ll get 瑠璃!
(I’ve been manually tracking that I’ve at least guru’d all these kanji… and of course, these helpful updates will come right after I will have reached level 60. 
)