Here is the scenario. Two RAW images have different Adjustment Brushes applied to them. They are created individually on every image, and they have different settings and brushstrokes. With Auto Sync on, and the two images selected I will add a new Adjustment brush that I want to be synced between those two photos. As I make my first brushstroke, all Adjustment Brushes applied to my active image will copy to the other image, overriding and deleting all of its existing Adjustment Brushes.
I tried this with Graduated Filter, Radial Filter and Spot Removal Tool. They all override all existing edits with the ones from the active image.