Seen with serviceability/sa/ClhsdbFlags.java, which sets ParallelGCThreads=1. Then Shenandoah heuristics acts and sets ConcGCThreads=2. Then Shenandoah adjustment kicks in, figures ParallelGCThreads < ConcGCThreads, and bumps ParallelGCThreads to 2, overriding the user setting and failing the test.

$ CONF=linux-x86_64-server-fastdebug make images run-test TEST=serviceability/sa/ClhsdbFlags.java TEST_VM_OPTS="-XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC"

hashCode = 5 default
]
 exitValue = -1

 LingeredApp stdout: [];
 LingeredApp stderr: [OpenJDK 64-Bit Server VM warning: Shenandoah expects ConcGCThreads <= ParallelGCThreads, adjusting ParallelGCThreads automatically
]
 LingeredApp exitValue = 0
java.lang.RuntimeException: Test ERROR java.lang.RuntimeException: 'ParallelGCThreads = 1' missing from stdout/stderr