Reproduces on m7a.16xlarge with EBS-backed storage, maybe that's EBS interaction:

$ CONF=linux-x86_64-server-fastdebug make test TEST=java/nio/channels/FileChannel/directio/DirectIOTest.java JTREG=REPEAT_COUNT=100 2>&1 | tee out

STDERR:
java.lang.RuntimeException: DirectIO is not working properly with read. File still exists in cache!
        at DirectIOTest.main(DirectIOTest.java:108)
        at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:103)
        at java.base/java.lang.reflect.Method.invoke(Method.java:580)
        at com.sun.javatest.regtest.agent.MainActionHelper$AgentVMRunnable.run(MainActionHelper.java:333)
        at java.base/java.lang.Thread.run(Thread.java:1575)

It is a bit weird that the prior "write" test passes always: the file is not in cache when immediately after write. Maybe there is some read-ahead going on when file is reopened.