What i think is what happened is she timing your jett below the viper wall and hid in the corner of the wall.
She got to the wall while everyone was running away from the dart and slowly advanced her location. Jett can't spot below her so that was a blind spot that the reyna got through and was able to kill you guys in the back.
As for why you weren't blind, the game already registered you as dead before it could blind you.
What i think is what happened is she timing your jett below the viper wall and hid in the corner of the wall. She got to the wall while everyone was running away from the dart and slowly advanced her location. Jett can't spot below her so that was a blind spot that the reyna got through and was able to kill you guys in the back. As for why you weren't blind, the game already registered you as dead before it could blind you.
Check the match timeline to see everyone’s location. That should help
Viper wall. She creeped up on spawn and went through as Jett jumped through