I have close the ticket because one problem is mine (I don't see that I must click on image for apply design) and then I insert a sample description text (Lorem ipsum dolor sit amet, consectetur adipiscing elit) in image description and that work.
But with my original text don't work
After some tests I realized that the problem are the italian special characters as
à è ì ò ù
if one of them is present in the description text, it are truncated.
example: If I write
il tempo è bello in image description it save: il tempo