Observed once with in JDK 11 b21 same binaries run. There is no much info in jtreg output:
---------------------------
command: main RegisterDuringSelect
reason: Assumed action based on file name: run main RegisterDuringSelect
Mode: agentvm
Agent id: 42
Timeout refired 1200 times
Timeout information:
--- Timeout information end.
elapsed time (seconds): 1217.974
---------------------------

But if we look at jstack out, it shows stacks below snippet:

"SameVMThread" #2249 prio=5 os_prio=0 cpu=0.00ms elapsed=1209.40s tid=0x0000007d2dbda000 nid=0x2cd8 waiting on condition [0x0000007d28b4e000]
   java.lang.Thread.State: WAITING (parking)
at jdk.internal.misc.Unsafe.park(java.base@11-ea/Native Method)
- parking to wait for <0x00000000e0434208> (a java.util.concurrent.FutureTask)
at java.util.concurrent.locks.LockSupport.park(java.base@11-ea/LockSupport.java:194)
at java.util.concurrent.FutureTask.awaitDone(java.base@11-ea/FutureTask.java:447)
at java.util.concurrent.FutureTask.get(java.base@11-ea/FutureTask.java:190)
at RegisterDuringSelect.main(RegisterDuringSelect.java:112)
at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(java.base@11-ea/Native Method)
at jdk.internal.reflect.NativeMethodAccessorImpl.invoke(java.base@11-ea/NativeMethodAccessorImpl.java:62)
at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(java.base@11-ea/DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(java.base@11-ea/Method.java:566)
at com.sun.javatest.regtest.agent.MainActionHelper$SameVMRunnable.run(MainActionHelper.java:229)
at java.lang.Thread.run(java.base@11-ea/Thread.java:834)

"pool-3-thread-1" #2250 prio=5 os_prio=0 cpu=0.00ms elapsed=1209.35s tid=0x0000007d2dbd7800 nid=0x3894 waiting on condition [0x0000007d337be000]
   java.lang.Thread.State: WAITING (parking)
at jdk.internal.misc.Unsafe.park(java.base@11-ea/Native Method)
- parking to wait for <0x00000000e04202c8> (a java.util.concurrent.Phaser$QNode)
at java.util.concurrent.locks.LockSupport.park(java.base@11-ea/LockSupport.java:194)
at java.util.concurrent.Phaser$QNode.block(java.base@11-ea/Phaser.java:1127)
at java.util.concurrent.ForkJoinPool.managedBlock(java.base@11-ea/ForkJoinPool.java:3128)
at java.util.concurrent.Phaser.internalAwaitAdvance(java.base@11-ea/Phaser.java:1057)
at java.util.concurrent.Phaser.arriveAndAwaitAdvance(java.base@11-ea/Phaser.java:679)
at RegisterDuringSelect$1.call(RegisterDuringSelect.java:54)
at RegisterDuringSelect$1.call(RegisterDuringSelect.java:44)
at java.util.concurrent.FutureTask.run(java.base@11-ea/FutureTask.java:264)
at java.util.concurrent.ThreadPoolExecutor.runWorker(java.base@11-ea/ThreadPoolExecutor.java:1128)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(java.base@11-ea/ThreadPoolExecutor.java:628)
at java.lang.Thread.run(java.base@11-ea/Thread.java:834)


SameVMThread indicates that program has entered the last sentence:
        // ensure selectLoop completes without exception
#112 result.get();

But all println messages are not shown. There is also no exception observed.