They’re probably trying to see if there is a market willingness to pay for F by testing various price levels relative to J. No need to give it away just yet, they can always oversell J and roll up.

26J probably too much for everything other than JFK-LAX/SFO. Even UA domestic 772 is only 28J, and they have to really cut prices on the 28J 752 on BOS-SFO to sell those seats. I often buy for $400 ew with enough advance purchase.