The sleep status effect does not work on any version of Mugen Souls Z including the JP and KR versions.
I was trying to figure out why Slumbering Bosses wouldn't work so I decided to see if enemies were even able to fall asleep but it looks like it doesn't work. Enemies also can't make your characters fall asleep.
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List of glitches found in Mugen Souls Z. Bugs that appear in the game that are advantageous to...