Armor drops my AC.
 

I created a 15th level Wizard and gave her the FIghter dedication.

I whipped up some nice mithril armor for her, but when I equip it, her AC actually goes DOWN by 2.

Why does this happen?

In Classic I could attach a por file. What's the equivalent option for this new Herolab?

It's a mystery to me. As far as I know, all you can do is attach a PDF of the character.

Data export is on the roadmap, but hasn't been implemented yet. Submit a bug report with as much detail as you can.

I did a partial level 15 human wizard build. Selected the fighter dedication class feat and equipped a master quality mithril chain shirt. I am seeing AC go up by 2 when equipped.

Not sure what is happening for your build. On the Play tab you can click/tap on AC to bring up more details. You can see the breakdown of the calculated AC. Check there for more insight into what is happening.

I suspect they have access to your character as long as they know the account and character name.

Did you create custom armor or use standard armor with the "mithril" special material option added?