I agree with gan. As ccc raised, non original (we called OEM) brake pad, their quality could not be good. Low quality (which I called cheap models) often will cause this sound, or even cracked. You can spend $40 to change for a better quality pad yourself, or buy a few dollars brake squeal (type of brake cleaner) and spray on it. This is the cheapest workaround solution.
If the material is uneven, then this could either resolved as it wear off, or reoccur next time. My advise is to remember this brand so that don't use it again next time. However, I don't think you know what brand of brake pad you used. So just keep this in mind and ask the technician next time