This is what I'm referring to by the way and this is a straight multi-city search on DL.com (will book through Concur but it's reflecting the same thing).
Routing is RDU - DTW - ICN - BKK // SIN - ICN - MSP - RDU.
It's actually cheaper for me to do RDU - BKK ($600) and then separately book KE for SIN - ICN ($600) and then do a 3rd reservation for ICN - RDU on DL metal ($1000) which comes to $2200. For some reason at least on my dates (Friday or Saturday in mid-October) it forces SIN - RDU into a Y fare which blows pricing out of control and I'm just confused why DL hasn't added a through fare on SIN - RDU that books into the usual lower fare buckets.