I was there for both evening/night. I couldn't find her for some rea
son. 
Just checked it out, and it's not a bug, but an oversight in planning. You didn't get Rena's affection level high enough before proceeding with the main quest. In short, the quest expects you to be friends with her by this point.
It's nothing critical though. You can meet her in Thula's outskirts, the semi-random zone with the entrance West from Thula's inner map entrance. After getting to know her, she will start appearing in the tavern during evenings.