- Negative dex mods do not reduce the armor class while wearing armor. The whole idea behind dumping the dex stat is that you can use armor to make up for it. Plate provides 18 AC even with 8 in dex... not 17 due to the -1 score.
This is by 5e design, Heavy Armor specifically does not take into account your Dexterity Modifier, either it's bonus or penalty
