System Information
Operating system: Windows 10
Graphics card: NVIDIA GeForce 780 ti OC Guru

Blender Version
Blender 2.8 Beta
Date: 2018-12-03 00.27
Hash: a205493426b
Branch: blender 2.8

Short description of error
Can't weight paint directly on mesh after changing pose, can only paint in the empty area where it was originally positioned.

• Created and sub divided mesh with (CTRL+R),
• Add and extruded armature bones,
• Automatically parented the object with the bones,
• Posed the mesh,
• Switch to Weight paint mode.