Isn't the problem that there is more than one way to calculate natural armour in 5e? I'm AFB right now, but...
1. Draconic Sorcerer offers an alternative base when not wearing armour (a formulation to avoid making things stack). This makes natural armour work like other AC-base-altering abilities, such as Mage Armour, as well as AC-calcuation-altering abilities such as the monk and barbarian's Unarmoured Defense.
2. Warforged offers a flat bonus to AC, regardless of armour (in this case +1; though at the back of the DMG I believe Lizardfolk are presented as having a +3 Natural armour bonus). In these cases, assuming the DM allows it, appropriately-sized armour would change the base to which natural armour would be added.
3. Then there's barkskin, which should be working like natural armour, but whose poor wording has been well documented. I'll leave this one out of consideration here.
This inconsistency between 1 and 2 means that there is no single answer to this question -- it simply isn't one or the other. I'll note that all of the examples in 1. are in the PHB, and the examples in 2 are not; but still --
here's a little test:
a. Should a Warforged monk be harder to hit (and do damage) than a human monk, all other things being equal?
b. Should wearing appropriate-sized leather armour make a Lizardfolk harder to hit?
c. Are you willing to bend bounded accuracy to accommodate natural armour?
If you answer "yes" to two or more of these questions, then a flat bonus (a la Warforged) is your answer!
1. Draconic Sorcerer offers an alternative base when not wearing armour (a formulation to avoid making things stack). This makes natural armour work like other AC-base-altering abilities, such as Mage Armour, as well as AC-calcuation-altering abilities such as the monk and barbarian's Unarmoured Defense.
2. Warforged offers a flat bonus to AC, regardless of armour (in this case +1; though at the back of the DMG I believe Lizardfolk are presented as having a +3 Natural armour bonus). In these cases, assuming the DM allows it, appropriately-sized armour would change the base to which natural armour would be added.
3. Then there's barkskin, which should be working like natural armour, but whose poor wording has been well documented. I'll leave this one out of consideration here.
This inconsistency between 1 and 2 means that there is no single answer to this question -- it simply isn't one or the other. I'll note that all of the examples in 1. are in the PHB, and the examples in 2 are not; but still --
here's a little test:
a. Should a Warforged monk be harder to hit (and do damage) than a human monk, all other things being equal?
b. Should wearing appropriate-sized leather armour make a Lizardfolk harder to hit?
c. Are you willing to bend bounded accuracy to accommodate natural armour?
If you answer "yes" to two or more of these questions, then a flat bonus (a la Warforged) is your answer!