My repair shop has encountered a few issues regarding the front camera and proximity cable. There is the possibility that the original flex cable was stressed during removal or installation. There seems to be hardware compatibility issues, specifically with the iPhone 7 models, with some of the flex cables produced by third-party manufacturers, as we tried a flex cable from another supplier (it cost almost twice as much as our normal cables) but it solved the issue. I'm not 100% certain this is the same issue for you, but this has solved the problem we had with Cam&Proximity cables.