'Is implemented by' icon not shown for methods in the JDK jar

As far as I remember, this never happened, the icons only showed up for project
files, not libraries.

JDK 5

sometime around three builds ago, I've noticed that when navigating
into JDK methods, the icon above indicating that implementations
exist, does not show on the left.

What changed? Is there any way to turn it back on?

Cheers,

Bonny