We have lost LOADS of bristles on our stiff sill brushes, they are awful, also on the medium ones we have lost a lot, nowhere near as much as the stiff ones though.
Spoken with Gardiners, and it's been diagnosed as wear and tear, even though they are about 2 months old, and can no longer be used.
The medium trim ones have been used for easy 6 months and have lasted a lot better.
The old standard brushes lasted a lot longer and although not as great for sills, I feel better value and we will probably revert back to them.