[RPG] Is it a bug or intended that Unarmored defense from barbarian and monk do not stack with each other

I am not yet really familiar with the fifth edition, but as far as I know, it seems important to me to also consider that +2 WIS is also a boost to your save and some skills which can drastically affect your survavibility.

Instead of giving you my very own answer which may not be your depending your style of play, I will give you list of pros for each option.

Pros to +2 Wis

• +1 to perception checks means that you will less likely be surprised by a sneaky character, which makes quite a difference for a squishy character.
• +1 to save is always good to take even more at low levels... Yet it is a weak save for you, so at lvl 20 I doubt it will make a huge difference.
• +1 AC is pretty good, it ends up as a 5% miss chance which, on the long run, is better. Well, before lvl 20, enemies tend to hit significantly harder than 20 pesky damages. It won't do much better than "buffering", at best, a single attack.

Pro to +2 Con

• It gives you +20 HP so about 21 % more HP. It is a good bonus to your survivability yet it won't save you from a bruiser's beating.

  CON 12: 8 (monk) + 3.5 * 19 (average sorcerer HP roll) +20 (Con) = 94
  
  CON 14: 8 (monk) + 3.5 * 19 (average sorcerer HP roll) +40 (Con) = 114
  

• +1 to one of your strong saves, which is good and probably better on the long run, as the system seems to encourage you to be good in your area of expertise and count on friends to compensate your weaknesses.

Bonus:
As told previously, +1 CON won't net you the best result in term of sheer HP bonus, if you are really looking for a boost, for your HP, the feat 'Tough' is a better alternative.

Sort of, but no.

Unarmored Defense does not provide a bonus to AC. It provides an alternate means of calculating it.

So, your druid has two AC calculations available to him:

1. AC provided by beast form.

2. 10 + Dexterity Modifier + Wisdom Modifier

You don't get to add the two together, because they both set AC to an absolute value. A brown bear has an AC of 11. It does not have a +1 natural armor bonus.

Likewise, a Monk using unarmored defense has an AC of 10 + Dexterity Modifier + Wisdom Modifier. He does not simply add Wisdom to his AC.

Your Druid can use Unarmored Defense in beast form, but he has to choose between that and the fixed AC of the form. For a brown bear, his options are:

1. 11

2. 10 + 0 (Dex) + Wisdom Modifier

The Unarmored Defense will probably give him better AC than the bear had, but less than a typical monk has (due to the low dexterity).

For more detail on 5e's armor notation, see this answer.

Natural Armor

Instead of using a flat AC of 11, you can use the bear's natural armor calculation. This allows you to calculate the bear's AC in cases where its dexterity or other stats change.

The armor calculation for natural armor can be found in the January 2016 rules answers:

Natural Armor: 10 + your Dexterity modifier + your natural armor bonus. This is a calculation method typically used only by monsters and NPCs, although it is also relevant to a druid or another character who assumes a form that has natural armor.

[...]

Similarly, a druid/barbarian who transforms into a beast form that has natural armor can use either the beast’s natural armor or Unarmored Defense [...].

In this case, you have the choice of either of two calculations:

1. 10 + Dexterity Modifier + Wisdom Modifier (Unarmored Defense)

2. 10 + Dexterity Modifier + Natural Armor Modifier (Natural Armor)

You don't get to mix and match between these two formulas; you have to pick one.

Thanks to Aviose for finding this.

See Also

Jeremy Crawford has tweeted about Unarmored Defense and Wildshape, stating that you must choose either the beast's AC calculation or the Unarmored Defense calculation.