System Information
Operating system: Windows-10-10.0.19041-SP0 64 Bits
Graphics card: GeForce GTX 970/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 457.09

Blender Version
Broken: version: 2.91.0 Beta, branch: master, commit date: 2020-11-19 18:09, hash: rBdca36a8ec922

Also broken: 2.92, 2.90.1

Short description of error
With weight paint mode using gradient tool with paint mask on, click view port and then click and drag to make a gradient deselects selected faces.

Exact steps for others to reproduce the error

1. Select default cube
2. Go weight paint mode
3. Select gradient tool
4. Enable paint mask (M)
5. Select one face of the cube by left clicking
6. Left click to viewport (nothing happens)
7. Then try to make the gradient
8. Result: deselects the face and does not paint any weight

Based on the default startup