Can't update to Java SE 8u181 from 8u171 on El Capitan
Solved. Basically, I forgot that I had to install not the JRE, but the JDK as a workaround for some LibreOffice bug. The JDK does not allow updating through the Java preference pane.

As soon as I manually updated the JDK, the Java preference panel said I had the correct, 8u181 update.