Operating system: Linux-5.4.0-48-generic-x86_64-with-debian-buster-sid 64 Bits
Graphics card: GeForce GT 630/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 390.138
Broken: version: 2.91.0 Alpha, branch: master, commit date: 2020-09-25 20:54, hash: rB83dc97ccc081
Worked: (newest version of Blender that worked as expected)
Short description of error
Grease pencil material slots don't auto-erase void slots above the first material
Exact steps for others to reproduce the error
Create a GP object with more than 1 material slots.
In edit mode, chose some vertices and turn them into a new object (shift p).
In object mode, select the new object.
The materials of the new object are placed in the slots as/where they where before - with void slots above them.