It doesn’t, but that's because, although this originated as a question (in my mind) about an actual trip, it morphed into a more general question about “what would happen if this scenario occurred?”

So any use of actual cities would be misleading, as (to take your examples) it’s not a question about travel between ORD, DFW, and STL - it’s a question about travel among any three cities when the tickets are on 2 PNRs and there’s IRROPs that threaten to upend it all.

But thank you for trying to be helpful (unlike some other posters).