a negative (or positive) Dexterity modifier should not penalize (or boost) AC with Heavy Armor.
here is what the 5e SRD says:
"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."
when i make a character with a positive DEX the AC is as expected
HOWEVER when i make a character with a negative DEX the AC is penalized
attached are two files that show this
(im super new and do not know if this should be in DATA, CODE or OS. sorry)
Mac 10.12.1, Java 1.8.0_102