Ran this quest several times over to study all the variables so far the conclusions I've reached are...
1. The completed quest glitch occurs if you talk to the Salarian and receive the UV lamps before beginning the Herbal Entrepreneurs quest.
2. Completing the Entrepreneurs quest is supposed to allow you to take a sample from the plants by "Examining" them (hold triangle for PS4). This is where the bug is as you can't interact with the plants beyond scanning them. Once you've collected the plant the Running a Fever quest is supposed to begin.
