I think it was my description misunderstandable.
I set the assignment with the possibility, for the student, to deliver both "online text" and a "pdf" file (containing 2 different texts).
And the student sends both of them (not one of them).
In this case the online text produces the described issues on the behaviour of the pdf visualizer on the left (that goes blank).
Today I have noticed that even if I modify the set up of the "online text" option (putting it on and off) after the students have delivered, the pdf visualizer functioning is affected (it does not work or it does work, depending on on line text setting)