version: 2.80 (sub 74), branch: blender2.7, commit date: 2019-06-11 21:45, hash: d93a7290e506, type: Release
build date: 2019-06-11, 23:26:36
This one is easy to reproduce:
- Add a Grease Pencil Object (the Monkey will do)
- Go to the object's material slots
- Remove a slot used by a stroke
This deletes the slot AND the strokes! This cannot be intentional. Mesh Material Slots do not behave this way, and neither should Grease Pencil Slots.