It is expected behaviour (across all ruby versions). And if you experiment a bit further, you'll see that you always access the same hash, no matter which key you use:
>> a[:a][:b] = 1
=> 1
>> a[:c][:d] = 2
=> 2
>> a[:d]
=> {:b=>1, :d=>2}
The way Hash.new with a default argument works is: If you do hash[key] it checks whether that key exists in the hash. If it does, it returns the value for that key. If not it returns the default value. It does not add the key to the hash and it will return the same default object (not a copy) every time.
To get what you want, you want to specify a default block instead. That way, the block will be executed every time you access a key that is not in the hash. Inside the block you can create a new Hash and set the key to "point" to that hash. Like so:
Hash.new { |h,k| h[k] = {} }
