This is unfortunately common on the 21 I don't know the cause but replacing the display fixed it and thankfully all my stuff was still there. Best of luck.

My screen replacement kit came with an adhesive cutout labeled as "G998 TAPE 3RD MIC HOLD TAPE". After much searching I was unable to find anywhere in these instructions or on the phone where it discussed using this adhesive. I am unsure where in the phone the adhesive goes. Clarification would be appreciated. I suspect it may go under the camera assembly but in these instructions that is removed whole with the motherboard so it may be an unnecessary item.

when reinstalling my kit I purchased came with new brackets that go around the antenna and have some adhesive on them. These instructions did not discuss or show replacing the brackets and I am not sure it was entirely necessary to replace the brackets, but I did after first warming the antenna up with a heat gun to separate the bracket from the antenna. This was done on both the left and right antenna and attention must be paid to ensure the right bracket is being replaced as there is a slight difference between the left and right bracket.