Japan Airlines is known to block off certain seats for its own elites (eg JGC, Plat, etc) when it comes to advance seat selection.
Also, advance seat selection is not a guaranteed service for everybody but rather an extra perk for select revenue fare (eg. full-fare Economy) or status holders (Saphhire, JGC or plus) on Japan Airlines.
In short, advanced seat selection is probable but not guaranteed on Japan Airlines, depending on your ticket/status; however, you can always try to check in online T-24 and select your own seats then. By that time, the only seats blocked off would be airport-controlled seats and not possible to get other than in person at the airport check-in counter.
Other than that, Japan Airlines is known for its amazing hospitality and service on-board; safe travels and enjoy JL!!!