I'm sure it's something else (unless 1.07 just plain screwed up with the Yukkuri King encounter rate), because I've been looking for them for at least 1h30 of playtime and I haven't seen a single one. I also just found out metal and stray yukkuri are still as common as ever on longshot areas, so either those places run on a different system or there's no global yukkuri timer.
Wild guessing tells me it could also have to do with the gears in the final area, as the last time I ran into Yukkuri Kings was a long time ago when I hadn't beaten those bosses.
[EDIT] Wait, more science just popped up. I decided to kill some monsters there because well, there aren't many better places to grind, and I ran into four Yukkuri Kings, from around 55:10 hours of gameplay until around 55:40, with each encounter being around 10 minutes apart from each other. I killed things for about 40 more minutes after that, but no more of those things appeared.
taking it easy is too hard
[EDIT2] Even more science! Found another king at around 57:40, then 57:50 and 58:00, and then nothing in the twenty minutes after that. Two hours of cooldown after 3-4 yukkuri, 10 minutes apart from each other? Who knows.