Hi cosmoservice
Leggi tutto link is manually added by you in zen tools module settings for blog module as you can see from screenshot below
Category blog items have read more link builded on core , while your module has the link to the article contained in the article title already , so this is not a bug but regular joomla articles and zen tools behaviours, if you want to have read more as blog items then you should change zen tools layout from layout tab.
Hope it helps