Spanish/Latin users, help a fanfic writer out, please, and drop irl diminutives of this name that you've encountered. So far, a certain fandom has been using Juli because no one's letting us in on the secret of making Spanish name diminutives. Apparently that's wrong, but the people pointing this out will not divulge what the right way is.
For the person who asked for diminutives, Julieta is usually shortened to Juli (pronounced sort of like *hoo*-lee), and occasionally Julie, like Julie Gonzalo, for example. Not sure why you were told Juli is wrong?