Three disk failurs within 15 months on same channel

I've had something similar in my NV+, albeit on a much shorter timescale. When I first bought it, I put a pair of 2TB WD drives in bays 1 & 2 and left the other 2 empty. After about 3 weeks, one of the drives started generating SMART errors and failed almost immediately. Instead of RMA'ing it to WD, I returned it to Amazon for a replacement with a new device (I don't trust the reliability of refurbished hardware as all the junk migrates to the spares loop). The second drive displayed almost the same behaviour, as did the third - each arrived "as new", well packaged and in a undamaged static bag. The fourth drive worked perfectly and hasn't missed a beat for the last 2.5 years.

These were "early life" failures, and I strongly suspect a batch problem because of the behaviour I saw and the way I sourced replacements. In the OP's situation, I think the ~9 month gap between failures is a bit long to be regarded as "early life", but the temperature difference in the bay doesn't seem significant and I think it's more likely that you've been unlucky with the replacement drives than there being a problem with your NAS hardware....