Just did a few random searches on the UA app and it was showing available J/F seats on the Seat Map. Same as it's always worked for me. Didn't matter if the initial search was for Economy or First.
To be fair, I have also noticed similar behavior on the UA website, but in completely different scenarios. Examples that I can think of are when you are searching for award or upgrade space and all J/F seats will be greyed out if there is no award/upgrade space available. But I haven't ever noticed this for normal revenue bookings.