Indeed it's due to special mechanics. First, I have (almost) no diplomatic options with Japan: I can't declare war, ally, vassalize or anything with Japan. Second, for outsiders the peace cost of Japan is increased with the territory owned by Daimyo's: it's not unusual to have an annex cost of 500% for OPM Japan.Maybe this is something I don't understand about DW, but since Japan is an OPM, why couldn't you vassalize them? Is it some special mechanic?
That's why this mission is so out of place: while it's fine for normal OPMs, Japan is a special case for which the mission doesn't work.