If NiFi is built using Java 11, the Advanced UI of the UpdateAttribute processor does not load the rules. The processor will still effectively process flowfiles according to the rules (if they were created in a flow from a different build, for example). However, the rules are not able to be displayed nor edited in the UI.
Work-around: build NiFI using Java 8. The resulting executable can be run within either Java 8 or Java 11 JVM without demonstrating the above issue.
This problem affects versions 1.13.0, 1.13.1 and 1.13.2, at a minimum. Earlier versions have not been verified yet.