Mithril armor - bugged or do I misunderstand it?

I want to put Mithril chain mail on my Thaumaturge cleric.  I know regular chain mail is Heavy Armor and would therefore put my cleric at Disadvantage.  The description for Mithril chain mail, however, says "Mithral is a light, flexible metal. If the armor normally imposes disadvantage on Dexterity (Stealth) checks or has a Strength requirement, the mithral version of the armor doesn't."  So why am I still getting Disadvantage on Acrobatics, Athletics, Slight of Hand, and Stealth?

Because you don't have training with Heavy Armor.

The penalty you're describing (disadvantage on d20 Tests using Strength or Dexterity) is the penalty for not having training with a particular category of armor. Mithral Armor doesn't remove that penalty; it removes the disadvantage on Stealth checks if the armor otherwise has it, and the 10 ft Speed penalty for not meeting the armor's Strength requirement.

If you want training with Heavy Armor, you can take the "Protector" option for Divine Order (instead of "Thaumaturge") or multiclass starting with a level of Fighter or Paladin. There's also a feat called Heavily Armored that provides it, but it's usually not worth spending a feat on.

Alternatively, there's a magic armor called Elven Chain which bypasses armor training requirements (but doesn't provide the other benefits Mithral does). Maybe that's what you were thinking of?

That part of Mithril Armor is referring to how some armor, like Plate, gives you disadvantage on Stealth Checks. The penalty from lack of Proficiency is entirely separate.

The penalty you're describing (disadvantage on d20 Tests using Strength or Dexterity) is the penalty for not having training with a particular category of armor. Mithral Armor doesn't remove that penalty; it removes the disadvantage on Stealth checks if the armor otherwise has it, and the 10 ft Speed penalty for not meeting the armor's Strength requirement.

I get it now.  Thanks for the insight. 

If you want training with Heavy Armor, you can take the "Protector" option for Divine Order (instead of "Thaumaturge") or multiclass starting with a level of Fighter or Paladin.

I'll have to take one of these options, thanks!