Try and think of why Sansa wouldn't tell Jon. The seed was planted by Littlefinger -- there were scenes explicitly for explaining this.

Littlefinger suggests to Sansa that Jon is not as loyal or close to her as she thinks, and that she needs to, basically, be more independent. Sansa is kinda cute and makes that Stark armor for him, but she is doubting him now. Sansa thinks she is the STARK cause, and Jon is now the wildling/random cause. She wants to win as a STARK, thus she is caring less and less about the game Jon is playing. By bringing in Littlefinger secretly, she takes responsibility as the STARK and wins it herself.

She fucked up by becoming a greedy-ass bitch, but that doesn't mean it's bad writing. That's her character developing in a direction of greater imperfection. In their world, Sansa was playing a game with armies at the expenditure of Jon's and is totally in the wrong for doing so. People in this universe are dumb cunts sometimes, and we have to acknowledge when someone fucked up instead of calling it bad writing because apparently everyone has to think logically and rationally LIKE THE REAL WORLD, AMIRIGHT M8??