Most likely this is a white-balance / phone image-processing issue triggered by the flash and mixed indoor lighting.

With flash enabled, the phone changed exposure significantly (faster shutter, lower ISO) and probably recalculated auto white balance. Brass is a warm, reflective subject, so the camera may have tried to neutralize that warmth, pushing the brass toward gray while shifting other tones pink/rose. Phone processing can also apply aggressive color correction when it detects flash and mixed light sources like fluorescent + LED.

A simpler explanation than “bad brass under flash” is that the camera’s automatic processing is being fooled by the scene.

What to try:

• lock or manually set white balance if your camera app allows it
• avoid mixing flash with fluorescent/LED room lights
• add a neutral reference (like white paper) in a test shot so auto white balance has something neutral to read
• try a different camera app or phone if the result looks like an unwanted filter effect

So the flash itself isn’t inherently making brass gray; the phone’s auto color/exposure processing under mixed lighting is the likely cause.