Hello,
I met the problem two years ago.
My watch was changed under warranty.
I meet again the problem. Only the back button.
The watch is no longer under warranty.
So I decided to take the watch apart. I didn't find any problems with the copper pins that make contact with the motherboard.
However, I saw oxidation between the steel button and the “4-pin dome switch.”
I cleaned all button / "4-pin dome switch" with WD-40 for electronic contact.
Everything works correctly 
I hesitate to put silicone grease between steel button and "4-pins done switch" to protect again water...
