There are a set of options indicating counts for the various GC thread types. If these valuse are set to zero (default), the values are determined ergonomically.
However, unlike the same mechanism in ParallelGCThreads, the ConcGCThreads and G1ConcRefinementThreads values are never updated and always shows up as zero when running with -XX:+PrintFlagsFinal
E.g. when running "java -XX:+PrintFlagsFinal -version" on a 32 thread machine, the output is
ParallelGCThreads = 23
ConcGCThreads = 0
G1ConcRefinementThreads = 0
but expected is
ParallelGCThreads := 23
ConcGCThreads := 6
G1ConcRefinementThreads := 23 (or whatever value determined ergonomically)