I wasted some time with this bug. From what I can tell, the bug was introduced with commit 0a9e67654a0f072601fb3a24d420c3c09ebfd843, when upgrading jenkins.version property from 2.60.3 to 2.138.4.
It happens because of commit ba33bd67cdaef87aba8a4e95dca8dcf108a7d73f , in 2.138.2, which added an interface StaplerProxy to class Run. This interface defines a method Object getTarget() with an implementation in class Run.
The Promotion class, which derives from Run, also has a method getTarget but with return type AbstractBuild<?,?>, so a synthetic method, with return type Object, is created by the compiler which invokes AbstractBuild<?,?> getTarget().
javap -v ./promoted-builds-2.138.2/WEB-INF/lib/promoted-builds/hudson/plugins/promoted_builds/Promotion.class
public hudson.model.AbstractBuild<?, ?> getRootBuild();
flags: (0x0001) ACC_PUBLIC
stack=1, locals=1, args_size=1
1: invokevirtual #9 4: invokevirtual #10 7: areturn
line 93: 0
Start Length Slot Name Signature
0 8 0 this Lhudson/plugins/promoted_builds/Promotion;
Renaming AbstractBuild<?,?> getTarget() to AbstractBuild<?,?> getTarget2() fixes the bug, but I have no idea of its impacts because of the @Exported annotation.
Note: the new Object getTarget() method in Run as an annotation @Restricted(NoExternalUse.class) but I don't understand its meaning.