Trace level logs for ext root scan that are (known) single threaded report as using multiple threads:

[87.104s][debug][gc,phases] GC(1412) Ext Root Scanning (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.1, Workers: 6
[...]
[87.104s][trace][gc,phases] GC(1412) CLDG Roots (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 6
[87.104s][trace][gc,phases] GC(1412) CM RefProcessor Roots (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0, Workers: 6
[...]

which is misleading.