IIRC Barkeepers friend is based on acetic acid, otherwise know as Vinegar. So it can leach the zinc out of brass. Unfortunately my Chemistry classes date back to college in the mid 70's so I'm working with memories that are a bit sketchy. However, as I recall it the process of zinc being leached out of brass is Galvanic Corrosion, an electrochemical reaction that takes place when dissimilar metals are exposed to a wet bath containing certain salts and acids. If you are using a stainless steel bowl in your cleaning the presence of that stainless steel will enhance the galvanic process taking place. BTW, every ultrasonic cleaner made features a stainless steel bowl for the bath.

Now, here is where things are a bit sketchy for me and my memory of the exact process may be a bit flawed. However, here is what I think is going on. That is that the Zinc and Copper in the brass are functioning as an Anode and the stainless steel bowl is the cathode. What is happening is that both metals are being dissolved into the solution but because zinc is far more reactive than copper the copper that is dissolved in the solution precipitates out, thus forming that pink or red dusting on the brass.

The good news here is that it takes a lot of time before there is a real loss in strength in the brass. So, a single 10 or 20 minute bath wont have much effect. However don't make it a habit.