System Information
Operating system: MacOS Sierra Version 10.12.6, MacBook Pro (15-inch, Late 2011), 2.4 GHz Intel Core i7 Processor, 16 GB 1333 MHz DDR3 Memory
Graphics card: AMD Radeon HD 6770M 1024 MB, Intel HD Graphics 3000 512 MB

Blender Version
Broken: v 2.82
Worked: v 2.8 (or earlier)

Short description of error
For some unknown reason, the Strength tool in Grease Pencil doesn't work correctly when you subtract from the stroke's opacity. Addition works fine, but I can't get the subtraction working properly.
Exact steps for others to reproduce the error

1. Create any sort of Grease Pencil object, but make sure the Stroke is checked in the Material Properties box.
2. In Sculpt Mode, make sure the Strength Tool is active and either enter the Active Tool and Workspace Settings or hold the control key. If you use the former method, change Add to Subtract. Afterwards, simply attempt to brush the strokes.