jsonb concatenation loses properties on nested objects

I'm not sure if this is by design or not but I find it strange and believe
it's a bug.

If you concat two jsonb docs together, all first level properties are added
or modified.

Given:

select '{"FirstName": "Phillip"}'::jsonb || '{"LastName": "Haydon"}'::jsonb

You will get a result of:

{"LastName": "Haydon", "FirstName": "Phillip"}

However, if you have an object value with properties in that object that
differ between docs. The nested properties are lost.

select '{"User": {"FirstName": "Phillip"}}'::jsonb || '{"User":
{"LastName": "Haydon"}}'::jsonb

This results in:

{"User": {"LastName": "Haydon"}}

Based on the result of the first example I would have expected:

{"User": {"LastName": "Haydon", "FirstName": "Phillip"}}

Is this by design or is it a bug?

Thanks.

--

Kind Regards,
Phillip Haydon
www.philliphaydon.com