I've noticed of late that the term preposition phrase is being preferred to prepositional phrase.
Is this a movement or an idiosyncrasy?
The people who started the use of preposition phrase appear to have done so on purpose, as snailboat explains in the answer to which she links. My impression is that wider use has been not so much a movement as bare movement, sort of like Brownian movement. Perhaps we could call it a drift.
My own opinion is that using both X phrase and Xal phrase, with distinct meanings, is a useful tool. The general principle involves a distinction between structure and function. That is, we (meaning me, now that I have grokked the idea) use X phrase to designate a sentence constituent which is built on an X (headed by X is the jargon), and Xal phrase to designate a sentence constituent which acts as an X.
Consistently drawing this distinction would avoid the sort of confusion which is evident in the commentary to this question (which I suspect is what prompted your question). In the sentence “Jane is in the meeting room” the phrase in the meeting room is structurally a preposition phrase; functionally, however, it acts either as an adverb according to traditional grammar or as a predicate complement according to newer grammars, and would accordingly be called either an adverbial phrase or an adjectival phrase, depending on your sectarian posture.
What then would a prepositional phrase be? That's illustrated by the other example sentence in that question, “Jane, along with other students, is in the meeting room”. There’s room for all sorts of entertaining theological controversy in that sentence, but I’m sure that some people would regard the collocation along with as a single consituent acting as a sort of compound preposition. That would be properly described as a prepositional phrase.