I've had issues post-covid adding a domestic JL segment to a Y ticket. For some reason IME this is only an issue when all Y, but not if you are on F/J in the TPAC segment and Y on the domestic segment.

In these cases I haven't been able to add the domestic segment when the TPAC segment is on hold, or add the domestic segment after ticketing. I end up buying the domestic segment with avios or cash. UA miles on domestic ANA are also competitive, 5-6k per segment, one of the few corners of value left on that program.