Experiencing the same issue as someone else with trying to book a connecting itinerary in first class using the companion ticket. Fare class Z and I is available when searching for a round trip flight on Delta's site, but when running a search with the companion ticket, I don't see any first class options.

Called twice, both reps unable to book. Chat two times, first rep said unavailable, 2nd rep was also unable to book, and transferred me to global ticketing support.

Gave the global ticketing support agent the flight numbers and said that Z is available on the outbound segments and I is available on the return segments. They said it is unavailable despite Z/I being available, because the lowest main cabin fare being sold on the same flights is Q. Here is the exact wording:

"Right but L has to be available also in main cabin in order to use first class. The system will only give you options that are available for companion fares when you are booking it online. If it doesn't go through that means it is not available."

Is this a true statement from the global ticketing support chat agent?