Dear Frabrizio,
Really sorry for the big delay. We are very busy with WPML 3.1.6 release(planning to release it tomorrow) and not get much time to spent on your issue. Today I have spent some time on it.
I have updated Types to 1.5.7 and tried with WPML 3.1.5(In my local copy). I can view the field table, after I choose "Translate" option. I can view the "Posti tappa" post as parent and "Itinerari" post is null. Please check attached image and let me know it is okay or not. And please correct me if I am wrong.
For your information, In image you can see English as a non default language. I have added it for changing the profile language, because I am not familiar with other languages.
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With Regards
Bigul