I have experienced the same issue with the new FP2 generation (bought June 2018).
The disassembling/reassembling “solved” the issue for my phone only for a couple of weeks, before I had the same issues (over) again.
Now I have found a “solution” that is stable for months which I would like to share here (maybe worth trying for some of you):
The main camera module is tightened by three screws: two screws at the electric contacts and one screw close to the camera lense. The latter (screw close to lense, see picture below) I did not tighten during reassembling - instead I kept it almost completely loosened (maybe you can also just remove that screw completely…).
This way I hoped that the electric contacts are more stable (without any forces bending the piece away from the contacts). For me, this works now reliably for months.
Hope this helps!