Not sure why it wasn't flagged by test-patch.
I decided to dig into this. At the time that pre-commit ran for
YARN-4248, there was an unrelated license warning present in HDFS, introduced by HDFS-9414.
Unfortunately, if there is a pre-existing license warning, then the mvn apache-rat:check build halts at that first failing module. Since hadoop-hdfs-client builds before hadoop-yarn-server-resourcemanager, it masked the new license warnings introduced by this patch. This is visible here if you scroll to the bottom and notice module Apache Hadoop HDFS Client failed, followed by skipping all subsequent modules.
Maybe we can do better when there are pre-existing license warnings, perhaps by using the --fail-at-end option to make sure we check all modules. I filed