Okay, so we still get this problem sometimes.
We have like 200 pieces of the filter with the foam on it.
We saw some similar problems between the phones:
There are 5 different versions.
See the IMEI number (the first numbers)
0124, 0125, 0126, 0127, 0128
The strange thing, is that all versions have this UV filter with foam already installed, only the 0127 never has!
So @Chris, you say: Top Left - Filter removed (This is standard as it leaves the factory)
You're IMEI starts with 0127! Because other iPhone 4's always have this filter standard from factory!
Still have sometimes the problem after repair that the sensor is not working, when we just replace the complete flex with a new uv lens filter, it works 9 of the 10 times.