Probably because LWJGL recently switched to OpenAL-Soft.
I've never found OpenAL panning to be very reliable. Even before 2.8.4 I would get a very harsh "snap" (on my Mac) when the position would be near center. For reliable and smooth left/right panning you might want to look into a software mixer like TinySound. Another idea, which is more of a hack/workaround (although I have had good results on a couple of machines) is to play the sounds in sync on two different sources, one positioned left of the listener and one positioned right, and then emulate the "panning" by changing their volumes. This lead to a much smoother panning on my machine, but as I said it's an ugly hack and not a proper solution.