Originally Posted by
pythonisman
Not a mixed class fare, but I recently booked FCO-LHR-SIN, SIN-LHR, LGW-TRN in business.
The longhauls are in I and the shorthauls were in the J fare bucket, which surprised me when I booked it (but the price was still good).
Expertflyer is showing FCO-LHR as J9 C9 D4 R9 I9 and LGW-TRN as J9 C8 D4 R9 I8 so there's plenty of availability in I, wasn't sure why it went the way it did.
Your particular case is completely normal. The shorthaul feeders book into J to ensure availability to sell a longhaul itinerary in I.
As for the OP, I’m not entirely clear of the question since giving the exact airport codes will help with the answers.