After about a year of ownership, I was changing the oil on my 2015 and noticed a large area of "wet" (no dripping) on the transmission pan. At that time, there was online chatter about rear oil seals in the Cummins. The finger swipe test made it look like engine oil. Under warranty, to the dealer, it went.
I'm one of those lucky guys that has a good dealer with a good diesel guy. He told me he found two leaks, one was the engine oil pan gasket, the other was the transmission pan gasket. He repaired both and five years later it is bone dry down there.
I don't know if ignoring the "wet" is a good idea since it could get worst and present a major problem at the most inopportune time.