I don't think you can appeal to some sort of trickery based on expectations here. There is nothing to indicate tis adventure is designed for experienced players more so than new ones -- in fact, it's linear structure alone seems to favor the latter. Moreover, the Starter Set also has an encounter with a too powerful dragon, again with special rules regarding breaking off after taking significant damage. That scenario is explicitly aimed at new players. Given the similar uses in both modules, it is clear WotC is trying to convey something specific about dragon to 5E players (regardless of their previous level of experience with D&D): dragons are dangerous, but they also value their lives. That, all told, is a very good thing. Less fighting to the death on either side of the screen is a good thing IMO.
Yes, the one for Starter Set players breaks off after 68 points of damage, the first main adventure breaks off after 24 points of damage.
Both dragons do 12D6 with their breath weapon.
In the Starter Set, one of the PCs is set up to want to attack the dragon and their are other indications that the PCs should attack the dragon. The players are encouraged to attack.
I have no problem with adventure design with a monster that is just too powerful for the PCs to attack. I have a problem with adventure design with a monster that can knock PCs unconscious or kill them in round one if they fail a saving throw (and go unconscious even if they make the saving throw in many cases).
That's bad encounter design, I do not care who the author is and I do not care if WotC wrote the product. Just because WotC put it down on paper does not make it a good idea. A simple mistake on the part of the DM like the OP and a lot of time and effort is wasted.
I know that the players in my group took 2 hours each to create their PCs. They read through a lot of decision points. People are just not yet familiar with Backgrounds and the revised spells and races and such.
PC death should be due to extremely unlucky dice rolls, stupidity on the part of the players (which I do not agree attacking a dragon is after 14 years of D&D being that way), or due to some extremely heroic sacrifice. It shouldn't be a likely outcome given the most obvious set of actions for many players in the first two adventures designed for the new edition. That's f-ed up.
PCs are the protagonists and their lives should not so cavalierly thrown into the waste basket because a game designer wants to prove to everyone that the new version of dragon is not supposed to be messed with. I have no problem with dragons being super uber fighting machines, I do have a problem with the designers throwing those super uber fighting machines at 3rd level PCs. That's stupid encounter design, even for the encounter here that has the dragon run away so quickly. Not every DM (like the OP) necessarily reads or remembers everything written down in an adventure module.
DMs are human and it is not that hard to make a simple mistake.