ThreadInfo.from(CompositeData) returns a new ThreadInfo, choosing the fields to instantiate based on whether the CompositeData contains the fields in a particular JDK release. So, for example, if the CompositeData contains the fields for JDK 6, the fields for JDK 6 will be instantiated. Here is the relevant code:

       // 6.0 attributes
        if (ticd.hasV6()) {
            lock = ticd.lockInfo();
            lockedMonitors = ticd.lockedMonitors();
            lockedSynchronizers = ticd.lockedSynchronizers();
        } else {
            // lockInfo is a new attribute added in 1.6 ThreadInfo
            // If cd is a 5.0 version, construct the LockInfo object
            // from the lockName value.
            if (lockName != null) {
                String result[] = lockName.split("@");
                if (result.length == 2) {
                    int identityHashCode = Integer.parseInt(result[1], 16);
                    lock = new LockInfo(result[0], identityHashCode);
                } else {
                    assert result.length == 2;
                    lock = null;
                }
            } else {
                lock = null;
            }
            lockedMonitors = EMPTY_MONITORS;
            lockedSynchronizers = EMPTY_SYNCS;
        }

        // 9.0 attributes
        if (ticd.isCurrentVersion()) {
            daemon = ticd.isDaemon();
            priority = ticd.getPriority();
        } else {
            // Not ideal, but unclear what else we can do.
            daemon = false;
            priority = Thread.NORM_PRIORITY;
        }

The problem with this is that ticd.isCurrentVersion tests to make sure that *all* of the fields are present. Two of the fields - lockedMonitors and lockedSynchronizers - are spec'd to be optional. If the CompositeData is missing those two fields, the daemon and priority fields will not get set.

There is a test for this use case, but is assigns false to daemon and Thread.NORM_PRIORITY to priority, so the failure wasn't detected.