User blog comment:Hayashi H/Spring 2016 Event Guide/@comment-82.113.98.103-20160517114027

Not sure if you've already seen this, but apparently the drop rate of Yuu in E-3 depends on how many U-511/Ro-500 you already have. Which would also explain the "0% drop rate if you had 2 Yuu/Shioi" bug. The drop off was working as intended, someone just accidentally made the drop too steep.

Here's the site in question

Data looks as follows

Hard: Medium: Easy:
 * 0 U/Ro: 2% to 3%
 * 1 U/Ro: about 1%
 * 2 or more: about 0.5%
 * Not much data. About 2% if you don't have U/Ro, about 1% if you have 1.
 * 0 U/Ro: about 1.5%
 * 1 U/Ro: about 0.5%
 * 2 or more: about 0.25%