Some mobs, (such as creepers, spiders pigs, etc.) aren't dropping items. this has slowed down the efficiency of my mob farms.
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We're tracking this issue in MC-205130, so this ticket is being resolved and linked as a duplicate.
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I'm almost 100% confident that mob drops are pseudo random, and mobs like creepers have a chance to not drop anything. It's basically all luck (other than looting on a sword).