A DESCRIPTION OF THE PROBLEM :
ThreadGroup.enumerate() sometimes returns a small value (eg. 1) when in fact there are many threads in the system. The problem is caused because the method

private int enumerate(Thread list[], int n, boolean recurse)

Sometimes returns 0 , when it should return n

See https://hg.openjdk.java.net/jdk/jdk11/file/76072a077ee1/src/java.base/share/classes/java/lang/ThreadGroup.java#l428

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
No steps. I think the description is clear


FREQUENCY : occasionally