Your prestige should never drop from an infiltrator action unless you are spotted. In fact, it is being spotted that causes your prestige to drop—because people find out that you, a prominent noble from a wealthy, well-known family (even if you're brand-new at the game, IC, you're still among the best-known families in the world), have been spotted engaging in dark and dastardly activities most properly assigned to those of lower birth.
If you do see conflicting messages like that, and can screenshot it, please do so and make a bug report.
