To reproduce, summon a glowing mob with transparent texture parts or holding an item.

Left: 19w38b, Right: 19w39a

The glowing shader itself (entity_outline.json) has not been changed. The problem is that what the game renders to the "final" buffer no longer maintains the entity's transparency, instead always having full opacity.

Final buffer in 19w38b. A phantom, and an armor stand holding a stained glass pane. Checkerboard background added by me.

Final buffer in 19w39a: