Probably won't get this fixed for the 1.15.1 release since my main focus for that is just getting the features reimplemented. But it looks like barrel item lighting is, for some reason, tied to the direction the player is looking in. Enabling "GUI lighting" before drawing the item results in a similar effect, but it behaves slightly differently

The issue seems to be caused by squishing down the item too far. My guess is that it messes up the normals or something along those lines.