If proguard is enabled for your application, this may be the case when static inner classes are removed. You can add properties to proguard to save an inner class that implements JobServiceEngine with the -keep command.
For example, if the class that implements JobServiceEngine is JobServiceEngineImpl, which is the static inner class of JobIntentService, add
-keep class android.support.v4.app.JobIntentService$* { * }
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