Unfortunately, yes; both the long-distance daytime trains between Austria and Italy charge this extra fee, which I’ve heard people on here say is something to do with the company that handles the Italian part of the journey not accepting Interrail.

Despite what it says on the Interrail website, reservation is not actually obligatory on these trains, but if you’re already paying the €10 surcharge you might as well also reserve a seat, as it is a popular route.

However, if you’re prepared to travel for an hour longer, you can actually get to Venice from Villach with an Interrail pass for only €3 with reservations, rather than €10 without. First take the 09:45 or 19:29 Micotra service to Udine (free reservation for passholders at the ÖBB website), then change onto a local train to Venezia S. Lucia (temporarily obligatory reservation due to pandemic, €3).

As @EdM  said on that route you need no reservation sadly the system of Railplanner marks it as reservation compulsory because of the Surcharge :/

If you travel from Germany/Austria with a Eurocity or Railjet in Italy you have to pay a surcharge of 10€ (you can pay the surcharge at the train, online or at the station) my experience is that not all conductors charge these fee so i paid when the conductor asked on the train as it´s the same price.
 

I understand better now ! Is it the same for Venice - Milan? I could very well pay the 10 € on the train and not reserve my seat (there is really no one on the trains for the moment) ? I want to make sure i don't get taxed on the train just because i forgot to payand so the seat reservation is not really obligatory?

no this worked just for the trains from Germany/Austria to Venice :/

For all other trains you have to get the reservation in advance as the trains are reservation compulsory and you risk huge fines when you get onboard without reservation on the Italian Highspeed trains