After your business on Aya is done, return to the Tempest, where you may just find Peebee having another comm-altercation with Lexi, where the former accuses the latter of viewing the crew as mere "experiments". The next time you return to the Nexus you’ll get a call informing you of Lexi’s absence, and encouraging you to look into it.
The very name of this quest kind of gives things away - head on over to the Vortex and you’ll find Lexi sitting at a booth near the bar. Chat her up and she’ll reveal the unsurprising nature of her drinking binge. Respond to her as you will and she’ll head back to the ship.