macOS 12.1
Illustrator 26.0.2

A brush is applied to an object, then its appearance is modified through the "Options of Selected Object" dialog box found at the bottom of the Brush Panel.
If later the definition of the original brush is changed — all the modifications done through this dialog box are completely wiped out.
I would expect these settings to be sticky.

For example, if I flipped a brush across or scaled it for one path and then changed the colorization method for this brush, I would still expect this particular path to be scaled/flipped in the same way.