System Information
Operating system: Windows-10-10.0.19041-SP0 64 Bits
Graphics card: GeForce GTX 1060/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 457.30

Blender Version
Broken: version: 2.92.0 Alpha, branch: master, commit date: 2020-11-21 02:54, hash: rB0eb096f40605
Worked: (newest version of Blender that worked as expected)

Short description of error
Try to weight paint with "mirror X" and "vertex group X" on, on a model with groups .L and .R, fails. It still paints on both sides for the selected bone. Activating "Auto-Normalize" makes it fail even more as it paints on the other side.

Exact steps for others to reproduce the error

• Open the file
• Paint of the side of the selected bone (fails)
• Activate "auto-normalize" option
• Paint of the side of the selected bone (fails)