I had this same issue. Doing a hard restart on the phone fixed it for a few weeks at a time. If it was a broken flex cable would resetting the phone fix it? If not what keeps causing this issue?
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=== Update (12/27/2018) ===
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I found that when making phone calls the loud speaker button was greyed out, that is an issue that is definitely covered by warranty regardless of the screen replacement.
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I don’t know if replacing my screen caused it, but my phone was within warranty so I took it in and they just gave me a replacement and said that the screen repair was unrelated to the issue
I had this same issue. Doing a hard restart on the phone fixed it for a few weeks at a time. If it was a broken flex cable would resetting the phone fix it? If not what keeps causing this issue?