I'm looking to do mixed award booking.

JFK-HND-SIN
SIN-HND-JFK

I am able to get to the booking screen if I do all segments in PY, or all in J.

However, when I choose the same dates, with the same flights, but include a single mixed class, I get "This service is not available for the specified itinerary. Please check the User Hints below and re-specify your flight conditions."

I've never seen this issue before when mixing J and F. Have not tried any combinations with Y.

All showing as available, no waitlist.

For example

J,J,J,J ✔️
PY,PY,PY,PY ✔️
PY,J,J,J ❌
J,PY,PY,PY ❌
PY,J,J,F ❌
J,J,J,F ✔️

Are there unique restricitons on PY?

Any idea what's going on here?