Interesting. I wonder if the fact that Apple has their own Wallet feature that basically does the same thing is why Delta hasn't put that functionality into their Apple Watch app.

Either way, I did have an interesting case like this a couple weeks ago. I didn't have the upgrade list issues like OP, but I could only add my connecting flight BP and not my first flight BP (that I needed for TSA at my originating airport) to Wallet. In that case, like OP, I was in C+ on the first flight (ended up clearing at the gate) and F on the second flight (upgraded about 4 days out).