Even when I use their booking change UI to change just the flight time of the outbound, I see funky stuff now that I think of it ... Note the modest "From +₤192" at the top ... then the actual prices for any of the options (there are more but all very much in that same ballpark).

So at least me doing open jaw/SJC doesn't appear to be the culprit.

I need to figure out how I can read the precise fare rules. (And stop being greedy on First class deals in the future..)