Java-Direct | heap support to replace portions

According to various sources (though not specifically mentioned in JavaDoc), ByteBuffer.allocateDirect allocates the memory off the main JVM heap. I can confirm that using Java Mission Control, seeing that the program that calls ByteBuffer n = ByteBuffer.allocateDirect(Integer.MAX_VALUE) does not use much of Java Heap memory:

However, this off-heap memory allocation stops working when one limits JVM heap memory. For example, when I run the JVM with -Xmx1g option, the allocateDirect call causes the following exception: Exception in thread "main" java.lang.OutOfMemoryError: Direct buffer memory. I do not fully understand how can this JVM option pertain to off-heap direct memory allocation, as - according to the documentation - the -Xmx option sets the Java heap space size. If I allocate the memory using getUnsafe().allocateMemory(Integer.MAX_VALUE); the memory is allocated successfully. My JVM is as follows:

java version "10" 2018-03-20 Java(TM) SE Runtime Environment 18.3 (build 10+46) Java HotSpot(TM) 64-Bit Server VM 18.3 (build 10+46, mixed mode)

Is this kind of behaviour between Xmx and ByteBuffer.allocateDirect expected?

EDIT: There seemed to be a (non-reproducible) bug in JDK 1.7 with the same behaviour as described above. So is this a bug?