Pseudopsyche
First Post
The dragon attack was tense (the DM really sold his "random" breath weapon targets) and when our monk critted with his trinket ("ancient arrow of elvish make ") we really believed we'd legitimately driven off the dragon.
You did legitimately drive off the dragon. As written, the dragon has very little invested in the attack on Greenest, and it explicitly does not consider tangling with adventurers part of the deal.
The dragon at least believes that this is a sandbox adventure. Upon contact with a fellowship of four or five ragtag humanoids from different races and with eclectic gear, it can't be 100% certain it's not on the wrong end of a not-level-appropriate encounter. With literally nothing to gain by fighting, it takes off.
I'd also like to point out that the adventure as written does not force the PCs to engage the dragon. As written, if left alone, it will continue to harass the town a while longer and than wander off, its shock-and-awe role complete.
Your mileage/DM may vary.