Missing protected page below heap with compressed oops which use narrow_oop_base.
It is caused by 6951686 bug which is fixed in HS19 only. Because of that bug (requested address is not used in special memory reservation) the heap is reserved above 32Gb virtual address ( 0x00002aaaae200000) but the VM code thinks it does reserved at requested address and it does not need nonaccess prefix (protected page below heap), so code in ReservedSpace::protect_noaccess_prefix() is not executed. But after heap initialization in Universe::initialize_heap() the narrow_oop_base is set based on heap address.
So the ReservedSpace code is missing the check that heap is not reserved at specified address.