A point of clarity, the dragon sorcs ability is NOT unarmored defense. Its called Draconic Resilience.
Now others may be right about its stacking because of the wording of the mechanic (maybe), but not because of the multiclass stacking rules
Mearls has said that nothing stacks unless it says it does. That's one of the founding principles of 5E design. And people have been whinging on about it for a year now.
Different spells DO stack, because it says so explicitly (PBR 81)
A shield stacks because it says so explicitly (adds 2 to your armor; PBR 44)
Natural armor (from features of class, race, background) don't stack because they aren't
adding to anything.
Worn Armor doesn't stack, because, just like natural armor, they are adding to anything.
but also, a specific rule on page PBR 9:
Some spells and class features give you a different
way to calculate your AC. If you have multiple features
that give you different ways to calculate your AC, you
choose which one to use.
The question of whether this applies to bonuses to AC is open, but based upon the general principles, the spell and one bonus from feature and one base from worn or feature sounds correct and in line with Bounded Accuracy.
However... on PBR4...
Ability checks, attack rolls, and saving throws are the
three main kinds of d20 rolls, forming the core of the
rules of the game. All three follow these simple steps.
1. Roll the die and add a modifier. Roll a d20
and add the relevant modifier. This is typically the
modifier derived from one of the six ability scores, and
it sometimes includes a proficiency bonus to reflect a
character’s particular skill. (See chapter 1 for details on
each ability and how to determine an ability’s modifier.)
2. Apply circumstantial bonuses and penalties. A
class feature, a spell, a particular circumstance, or some
other effect might give a bonus or penalty to the check.
3. Compare the total to a target number. [...]
Note item 2 - I colored the most important part...
A ___.
That means, at least to me,
A feature = 1 feature
A spell = 1 spell (but noting the exception in spellcasting rules)
A circumstance = 1 circumstance (but none have been given that modify rolls that I've seen)
some other effect = somthing else.
The implication of the "or" is that you're only supposed to get ONE of them.
By the literal text, in the most literal sense, you don't get to add multiples unless they're spells... So either you're adding the Con bonus OR you're adding the Spells' bonuses, but I suspect that may be a bit too literal.