Front camera is not working

My iPhone 6s front camera stopped working recently. I have not dropped or damaged my device. The weird thing is that occasionally my front camera shows that it seems to be working when I only open FaceTime to see my recent FaceTime history and I can see in the background my front camera working showing the blurred picture behind the call history. When I try calling on FaceTime it will actually show my picture but after a couple of seconds the picture freezes and turns green with horizontal lines. At this point the camera stops working and even restarting/shutting down the phone doesn’t solve the issue. Sometimes when trying to get the camera app to show the front camera it would show on the screen that the camera application was used by another app. I turned off camera use for all apps which did nothing. Also a reset of all settings, clean restore, and iOS upgrade from iOS 11 to 12 doesn’t solve the issue which makes me think it’s a hardware issue. My front camera and flash work, as well as the proximity sensor, microphone, and earpiece speaker. I thought it was a faulty camera module so I replaced the whole flex cable for the front camera, sensor proximity light, microphone, and earpiece speaker connection. The replacement was a success since everything worked except the camera, just as before. As I had the phone open with the device turned on to view if everything worked before closing the device I tried the front camera but it still did not show a picture. With the phone open with one hand holding the screen slightly up and the phone flat on my palm I reset all settings and opened my camera. At this point my front camera started working for a little longer than before but then did the same thing with freezing, turning green, and showing horizontal lines. I feel as though it has to do with something other than the camera replacement. I don’t know if holding the camera at that certain angle pushed something in place while reseting all settings. I’m not sure where to go from here. I don’t want to spend money on someone repairing it for me since I want to fix the phone to get a good sell value for an upgrade. Can someone with expertise please provide some answer as to why my camera still may not be working. Any help is appreciated, thank you!