I didn't try this myself just yet, but people have stated this being an issue within the Discord. If others have a Heavy Armor character with 8DEX please check and confirm.
https://www.5esrd.com/equipment/armor/ <- 5e SRD rules.
"Heavy Armor: Of all the armor categories, heavy armor offers the best protection. These suits of armor cover the entire body and are designed to stop a wide range of attacks. Only proficient warriors can manage their weight and bulk. Heavy armor doesn’t let you add your Dexterity modifier to your Armor Class, but it also doesn’t penalize you if your Dexterity modifier is negative."
I can confirm that the game gives a malus to AC based on low DEX even while wearing heavy armor.
So yeah, that's something to fix if the game wants to implement rules-as-written.
Seeing the same bug too.
Yep, my Dex-dumped paladin is feeling the pain on this one. AC 17 with Plate equipped.