Hi @DavideV ,
I suggest you check the outputs of the trigger to see if the author field is included?
Sometimes we can get the value of the field by expressions even if it not showing in the dynamic content, but if the field does not exist, we cannot get the value of the field.
I have made a test on my side, it seems that the author field does not exist in the dynamic content.

In addition, if the 'primaryLink' field contains some information about the author, maybe you could try to extract it as the value of author.
Best Regards,
Charlie Choi