That s because when forbidding a nickname, anope uses the ircd's provided method (q-line), but when forbidding a channel, anope handles it on its own. Maybe anope could be made to make use of the ircd's system, but i am not even sure that list can be modified dynamically. As far as i know it s statically defined in the config fle.
Either way, it may be an improvement, but definately not a bug