Just got my 3rd Fenix 5s replacement for this issue. I was told (this time - not in the past) that this is due to pool swimming and chlorine build up on the sensor and that I need to rinse it with fresh water after every swim and if I do that, it shouldn't happen.
The sensor is inside the right hole, so even you wash it is difficult to clean inside. I try also to put the watch into distilled water overnight weakly to try to dissolve impurities. But nothing fail happened anyway after 6 months. So maybe clorine oxide something
My Fenix 5s Plus died about two weeks ago, same issue. Getting a replacement, but disappointed the failure occurred. I don't think I swim that much - two times a week, three times, tops. I'll try rinsing the replacement as a preventative measure. For the $ paid, you would think the watches would be more robust