The jtreg test java/lang/instrument/IsModifiableClassAgent.javafails on macOS in our nigthly tests. It seem to have start to fail around February/March 2020.
I see the error on macOS 10.14 and 10.15.

Relevant part of jtr file:
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#section:main
----------messages:(4/246)----------
command: main -javaagent:IsModifiableClassAgent.jar IsModifiableClassApp
reason: User specified action: run main/othervm -javaagent:IsModifiableClassAgent.jar IsModifiableClassApp
Mode: othervm [/othervm specified]
elapsed time (seconds): 2.529
----------configuration:(0/0)----------
----------System.out:(2/66)----------
IsModifiableClassAgent started
modifiable: 591. unmodifiable: 120
----------System.err:(22/1671)----------
Error: bad return from retransform: class java.util.Locale
  ERROR: java.lang.UnsupportedOperationException: class redefinition failed: attempted to change the schema (add/remove fields)
Error: bad return from retransform: class jdk.internal.org.objectweb.asm.Label
  ERROR: java.lang.UnsupportedOperationException: class redefinition failed: attempted to change the schema (add/remove fields)
Error: bad return from retransform: class sun.invoke.util.VerifyAccess
  ERROR: java.lang.UnsupportedOperationException: class redefinition failed: attempted to change the schema (add/remove fields)
Error: bad return from retransform: class java.util.HashMap
  ERROR: java.lang.UnsupportedOperationException: class redefinition failed: attempted to change the schema (add/remove fields)
java.lang.Exception: ERROR: IsModifiableClass failed.
at IsModifiableClassApp.run(IsModifiableClassApp.java:39)
at IsModifiableClassApp.main(IsModifiableClassApp.java:31)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:564)
at com.sun.javatest.regtest.agent.MainWrapper$MainThread.run(MainWrapper.java:127)
at java.base/java.lang.Thread.run(Thread.java:832)

JavaTest Message: Test threw exception: java.lang.Exception: ERROR: IsModifiableClass failed.
JavaTest Message: shutting down test

STATUS:Failed.`main' threw exception: java.lang.Exception: ERROR: IsModifiableClass failed.
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