LR Classic 7.1 on Windows 10 v1709
Wacom Intuos Photo
Video Link: https://youtu.be/mVVVyYn0MTE
This problem does not happen in PS, so I think the issue is limited to LR.
LR for Windows though. I cannot reproduce the issue on my MacBook Pro running OSX 10.11.6
When I make small movements with the Wacom pen, the mask is not applied, it is not until I move a good distance away while still pressing that I get the mask applied.