Planning an SDC and doing a test F booking to get a ballpark idea. LAS-JFK on Sunday 6/12 with one stop.
There are a few results through LAX that say "Multiple Cabins" which I figured was because of First vs. Business but on a closer look the LAS-LAX flight says Main Cabin and "Upgrade Eligible" and even in some cases "Upgrade Available"
e.g. DL2808/DL40 (this is an F search so the left column is Delta One/First and the right column is -Flex):
DL761/DL1162, DL761 shows (B) and Upgrade Eligible with 5 F seats unassigned on the map
not just thru LAX either, LAS-SLC-JFK DL2140/DL1264, DL2140 shows (B) and Upgrade Eligible with 2 seats unassigned on the map (the other oddity on this one is that popup on the flight number or expanded Details show DL1264 as a 738 but View Seats pulls up the map as a 36J 763)
IT glitch? One of the samples is full Y/J so I wouldn't think it's a fare rules quirk.