Hi Juan,
Nice work on your progress so far!
Honestly, I'm not sure what might cause the inverted chest leads that you see. From the ADS1298's perspective, you appear to be using the device properly. Someone with higher medical expertise may advise you to derive the WCT and chest leads a little differently, that part I'm not an expert on.
From what I do know, it is not uncommon to see some leads look inverted from the others. If you think of each lead as a different "camera angle" viewing the heart's electrical activity, it makes sense that you see a difference as you move across the chest from V1 to V6.
If you do discover another reason, please share it. :)
Saludos,