I recently experienced this phenomenon. Routing is SFO-AMS-HAJ in J. Flight is not for another month. I had my booking changed to earlier flights after my original AMS-HAJ leg was canceled. I was able to select seats on both legs, however now the seatmap is non-accessible for SFO-AMS. This is true using ExpertFlyer as well (returns an error for J cabin, but Y seatmap shows). I think there were about 7 or 8 unselected seats in J when I chose our two seats. The bummer is I would've preferred 2 seats together in the center section, but now I cannot set an EF seat alert in case they open up.