I'm having trouble booking an award ticket with the first leg on AS and the second on BA. There is award inventory available on both flights (U space on BA in J and both economy and first fares on the AS flight feeding the BA leg).

However, multi-city searches won't display the two flights I want...the connection time is 1 hr 40 mins, so well above the MCT. I called customer service and they informed me that the "space isn't available, this is because BA is controlling the flights"...but every seat on the AS leg is available. They verified MCT wasn't the issue here.

After the agent did some more digging and told me it was actually that the specific fare code on the AS flight wasn't avail so BA wasn't allowing that flight to be added to their long haul leg. Problem is, agent said we need D, E, or I fares for the AS leg. I did a test checkout of buying the AS leg separately and lo and behold, it's showing a D fare if I use the miles for First. Main is showing N fares.

So not sure why I can't book the two together? I've also seen plenty of award tickets with the AS leg in Main (N or S) and the BA leg in J... Agent seemed to be fairly knowledgeable but told me they couldn't do anything to make the fare code available and that I'd just have to wait and call back in. While I could book them separately, it would likely require 15-20k more miles than the two legs booked together. Anyone had similar issues or workarounds?