System Information
Operating system: Windows 10 Pro 64-bit
Graphics card: NVIDIA GeForce RTX 2080 Ti

Blender Version
Broken: Blender 2.81 Alpha, September 21, 12:20:50 - ef45d1c8cd43

Short description of error
In Sculpt Mode, the Topology Automasking and 2D Falloff options don't work correctly when both are activated.

Exact steps for others to reproduce the error

1. Subdivide the default cube a few times until it is a sphere, by pressing Control + 2 or 3.
2. Apply the Subdivision modifier.
3. Enter Edit Mode, select all.
4. Shift + D to duplicate, move the copy until the spheres are a bit separated, but still intersect.
5. Enter Sculpt Mode.
6. Activate Grab brush.
7. Check Topology Automasking and Options ➔ 2D Falloff.
8. Try moving the spheres with the Grab brush. Sometimes one of the two spheres will not move at all, sometimes just a part of it will move.