It seems t default to the last setting you used before starting to brush the last time you brushed. This behavior annoys me too but is probably intended to be this way
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This applies to Brush, Gradient Filter and Radial Filter-
Double-click on the word [Effect], Close the Tool. Now the Tool sliders will be reset to zero.
There are now two situations that will change the slider defaults (Example for Brush)-
a) If you open the brush (sliders at zero) and brush an area (maybe with the overlay visible [O] ) then AFTER painting- change the sliders, exit the brush, the sliders will be zero when next used.
b) If you open the brush and set the sliders BEFORE brushing, then brush on the image, exit the brush, then the sliders will remain at the last used setting when next used.
I think many would just double-click [effect] each time the Tool is activated.
After all this time of using Lightroom I never took the effort to figure out when the brush setting "are/are not" saved and just reset them most of the time. Your explanation was nice and concise and easy to understand!!!