Watermark

Hello, I went in to T-Mobil the other day to purchase the new iPhone and trade my current one in. They did a couple checks and told me my water mark was missing and if they don’t find it- it fails the trade in test and I won’t get the trade in value for it. I had this phone completely replaced my T-mobil because I had insurance that covered a full replacement, I’m not sure what to do because it was a completely new phone that has not had water damage... I am not sure what the watermark represents or what it is supposed to do but for some reason they didn’t see it, Is there anything I can do about it?