It came out there was another problem (my fault!): there were almost 300 posts when I switched on the option “Select multiple values”, so I guess all the old posts still have a string in the field “tipo_lemma”, while the new ones have an array whether they have one or more type selected.
In fact, if I change the option “Return value” from “Label” to “Array” nothing happens, but when I open and save again the old posts, then it works. I just need to re-save all the posts then.
Thank you for the support! Your code works perfectly, so I’ll mark the question as solved 🙂
