I have 3 entity

A -> B <-> C

All assoc is OneToMany (from left to right),
the arrow indicate uni-dir or bidi.

For the B and C association, C is the owning side.

When persist a entity (a tree with around 4 level) and then
merged again.

The returned entity (Java instance) is the exact copy I used to persist.

When this seems reasonable.

However, when I access properties
List<C> something = a.getB().getC();

it is null.

But I expect I can get a enchaned version of B,
so getC will trigger lazy-loading.

Is my understanding wrong, or I have missed anything?

This is not the correct behavior but you don't give enough of the context to be helpful.
Try and repost using the question template.