By the way, just in case anyone is still following this, my current trip is SFO-DEN-LNK-DEN-SFO, and I saw a cheap F fare and snagged it. This is a regular paid ticket, but is "mixed cabin" since the DEN-LNK-DEN legs are in single-cabin CRJ200s. And again, all four legs have the notation NON_PREFERRED_CABIN, even the two in F on mainline metal (fare class A). As predicted, the notation was meaningless and it was a perfectly normal F flight. Just a bug I guess as predicted above.