There is a bug in 1.13 snapshots that causes item models of transparent blocks to behave strangely. Eventhough block models of transparent blocks have always behaved differently than models of non-transparent blocks, their item models behaved the same in earlier versions.
In these example screenshots I am holding a normal glass pane in one hand and a magenta stained glass pane in the other hand, both remodeled to the exact same item model. In the first picture, which was taken in version 1.12 both models look exactly the same. On the other picture, which was taken in 18w05a the stained glass model looks transparent, eventhough the glass pane model looks the same as in 1.12.