In Java SE 8, various additional information associated with parameters can be stored in a class file, including parameter names and modifiers. With attempts to update JDK_MINOR_VERSION in JDK 9, this code in src/share/vm/classfile/javaClasses.cpp
if (JDK_Version::is_jdk18x_version())
java_lang_reflect_Parameter::compute_offsets();
seems to improperly limited parameter support to JDK 8 rather than JDK 8 or later.
This is the suspected cause of java.lang.reflect.Parameter test failures in the jdk and langtools repos observed in test builds where JDK_MINOR_VERSION has been incremented to 9.