I would recommend you go to an Apple Store and with your original repair paperwork in hand (from the first time) and your watch to see what they say.
You might be having a chemical reaction with your sweat on your arm and the adhesive Apple used to hold the sensor disk.
See if you can get ahold of some ph test strips. Place one under the watches location and have a few friends do the same with their watches. After building a sweat on a jog or a game of basketball, see what yours looks like and your friends are they the same? While this only checks for how acid or base your skin is in comparison with your friends it might be a clue.
You may also want to see a doctor if yours is not the same as your friends as you could have an underlying health issue here as well.
In any case let us know what you find out.
Given the fact this is a new product category for Apple, you do need to keep that in mind. Thats not to say they get a pass here, far from it! They need to service the product until they get it right. Sounds like they've taken care of you.
While there are a few cases of a problem, I haven't seen a mountain slide of complaints online. So you are unique and Apple needs to learn about why the watch is failing on your arm and not others so it can be used by all. Remember Apple has sold over 12 million Apple Watches! So they appear to have met with what the public wants for durability.