I was told at a tournament that stealth device is discarded when a hit symbol is rolled no matter if I dodge it or not. That would mean the first time somebody shoots and rolls a hit would already cancel the effect. Even before I roll for defence.
Doesn't sound right to me. What's correct? I'd assume only if I fail to dodge and there is a real hit either loosing shields or getting a damage card would discard stealth device.
A "Hit" is well defined in the rulebook. If someone rolls attack dice and you roll evade dice and there are uncancelled hits remaining after you used the evade results and/or tokens, then and only then are you hit.
So whoever told you that at the tournament was very wrong.
However, loosing a shield or taking damage is not a requirement. So for instance if there is only one crit remaining and someone with "draw their fire" takes that instead of the targeted ship, then you are still considered hit and loose the stealth device.