The following tests all fail with loom:

vmTestbase/nsk/jdi/stress/serial/mixed002/TestDescription.java
vmTestbase/nsk/jdi/stress/serial/mixed001/TestDescription.java
vmTestbase/nsk/jdi/stress/serial/heapwalking002/TestDescription.java
vmTestbase/nsk/jdi/stress/serial/heapwalking001/TestDescription.java
vmTestbase/nsk/jdi/ObjectReference/referringObjects/referringObjects003/referringObjects003.java

The error is always something like:

nsk.share.TestFailure: List with wrong size was returned by ObjectReference.referringObjects(ThreadReferrence): 5, expected: 6

So it seem that loom is somehow introducing an extra reference to objects. Most likely this is a test bug that is assuming certain library references to objects that might no always be limited to what the test expects. A comment in the test says:

 * - check that threads and thread group have correct number of referrers:
 * (thread referrers should include thread group and references with supported types,
 * thread group referrers should include group's threads, parent thread group and references with supported types)

So I can certainly see the result of this being implementation dependent.