jcstress caught the error in current jdk/jdk. It proves hard to reproduce, though.

[VM ERROR] o.o.j.t.atomicity.varHandles.byteArray.big.GetAndSetTest.GetAndSetLong
    (JVM args: [-XX:+UnlockExperimentalVMOptions, -XX:+UseShenandoahGC, -XX:+UnlockDiagnosticVMOptions, -XX:ShenandoahGCHeuristics=aggressive, -XX:+ShenandoahVerifyOptoBarriers])
  Observed state Occurrences Expectation Interpretation
       -2, 0, -1 180,087 ACCEPTABLE actor1 execute after actor2
       0, -1, -2 431,724 ACCEPTABLE actor1 execute before actor2
        0, 0, -1 0 FORBIDDEN This won't happen because getAndSet is atomic
        0, 0, -2 0 FORBIDDEN This won't happen because getAndSet is atomic

    Messages:
        #
        # A fatal error has been detected by the Java Runtime Environment:
        #
        # SIGSEGV (0xb) at pc=0x00007f91d1a1ff90, pid=79746, tid=79969
        #
        # JRE version: OpenJDK Runtime Environment (14.0) (fastdebug build 14-internal+0-adhoc.jenkins.jdk-jdk)
        # Java VM: OpenJDK 64-Bit Server VM (fastdebug 14-internal+0-adhoc.jenkins.jdk-jdk, mixed mode, sharing, tiered, compressed oops, shenandoah gc, linux-amd64)
        # Problematic frame:
        # C 0x00007f91d1a1ff90
        #
        # No core dump will be written. Core dumps have been disabled. To enable core dumping, try "ulimit -c unlimited" before starting Java again
        #
        # An error report file with more information is saved as:
        # /home/jenkins/workspace/nightly/jdk-jdk/build/jcstress/hs_err_pid79746.log
        Compiled method (c1) 367 92 3 java.lang.CharacterDataLatin1::toUpperCaseEx (71 bytes)
         total in heap [0x00007f91b0ce7490,0x00007f91b0ce7b38] = 1704
         relocation [0x00007f91b0ce7608,0x00007f91b0ce7670] = 104
         main code [0x00007f91b0ce7680,0x00007f91b0ce7940] = 704
         stub code [0x00007f91b0ce7940,0x00007f91b0ce7998] = 88
         metadata [0x00007f91b0ce7998,0x00007f91b0ce79b0] = 24
         scopes data [0x00007f91b0ce79b0,0x00007f91b0ce7a28] = 120
         scopes pcs [0x00007f91b0ce7a28,0x00007f91b0ce7b18] = 240
         dependencies [0x00007f91b0ce7b18,0x00007f91b0ce7b20] = 8
         nul chk table [0x00007f91b0ce7b20,0x00007f91b0ce7b38] = 24
        #
        # Compiler replay data is saved as:
        # /home/jenkins/workspace/nightly/jdk-jdk/build/jcstress/replay_pid79746.log
        #
        # If you would like to submit a bug report, please visit:
        # http://bugreport.java.com/bugreport/crash.jsp
        #
        Current thread is 79969
        Dumping core ...
        OpenJDK 64-Bit Server VM warning: Option MaxRAMFraction was deprecated in version 10.0 and will likely be removed in a future release.
        OpenJDK 64-Bit Server VM warning: Option MinRAMFraction was deprecated in version 10.0 and will likely be removed in a future release.