It seems like the issue is itemization. On the one hand you have some medium armors that grant full dex mod to armor, and on the other hand the bonuses on heavy armor are not inspiring.
Full dex mod on medium armor is too strong, when you consider that it also gives initiative (as has already been pointed out). If this were reduced to granting an additional +1 or +2 to AC from dex then it would still be very strong without completely eclipsing heavy armor.
At the same time they need to put some sources of full plate earlier (act 1 for sure), and make full plate +1 accessible in act 2. Alternatively they could buff the damage resistance component. -1 or -2 to damage doesn't feel like a big deal, but a -5 or -10 would be impactful enough that I wouldn't mind getting hit.
