tnid not updatet when created new translation

Hi,
I don't know if this problem arises i18n or core module, but I try to ask you some hints.
My test site has two languages: italian (default) and english. I have defined one custom content type ('test') with translations available.
If I create a new 'test' node, everything works well (I can chose italian, english or neutral). If later I try to add a translation for this node (example: the first node is 'italian' and I try to add 'english' translation), the node is created but it doesn't "linked" correctly as translations of the previous node.
I have investigated a bit on the database and the problem is that the tnid of the first node (source) remains 0 also after the translation creation. If I change manually the tnid field of the source node (equally to the nid field of the same node), everything works well (for this node).
Someone knows a possible cause of this "problem"? At the moment I can edit the value in db, but obviously it isn't a feasible on a working/live site!
Thanks in advance,
Saxx