So Armour Class is calculated as:
10 + Proficiency + Armour bonus + Dex modifier + any other bonuses + any other penalties
with Touch Armour Class being calculated in the same manner.
Based purely off of text, this could easily result in a monk, being untrained in any armour, subtracting 2 from their AC to account for proficiency. Since I doubt this is what was intended, there should be a note made that the proficiency added to one's AC is the character's proficiency with the type of armour being used.
In addition, WHAT IS the proficiency bonus for a character not wearing armour supposed to be? As a GM I'd treat it as 0 so that's what I currently have it marked as for my monk, but I couldn't find anything to confirm that. (To be fair I'd been up for over 24 hours when I made my monk and at least 27 when I noted this so I may have just overlooked it).
Right underneath Flurry of Blows is the feature Graceful Expertise, which give monks Expert proficiency in unarmored defense. And the apparent default proficiency of characters other than monk is untrained since it isn't listed in their proficiencies so presumably you'd calculate your armor as having untrained proficiency.