ISSUE: An unknown combination of adjustments 'breaks' the new mask / brush mask tool, causing LR classic to make the brush size 'maximum size' regardless of what the brush settings are actually set to.
Steps to reproduce the bug:
1) Open a RAW file - In my case, raw from a Nikon D600 (Full exif on request)
(1a / OPTIONAL: Confirm the mask/brush tool is working as expected)
2) Apply the preset (made by me) "Frosty Caravan" (preset attached)
3) Attempt to use the brush tool. If the issue has been replicated you will notice it that regardless of whatever brush size, feather, strength you select, the brush will act as if all sliders have been set to max - size, feather, flow, etc - all act as if set to max, despite this change not being inputted to or reflected by the sliders.
Picture attached of a single click using the mask/brush tool - note the settings to the right, compare with the affected area visible in the mask overlay
EXTRA INFORMATION: THE PRESET
I did nothing special to create this, no outside editors or manipulation outside of LRC at all. I was working on a photo, liked the colour grading I had done on it, so decided to save it as a preset so that I could use it on other photos I took in that location/at that time easily.