Baldur's Gate : The Sword Coast Chronicles : NWN2 Persistent World

Didn't think using the Appearance Changer from the player menu to alter an adamantine breastplate would cause an issue since from what I've read mythril armor was the one that experienced issues, but it seems that after some tweaking it no longer has the medium armor 2/- DR. All other properties remained (it still lists Material as Adamantine and has kept its +3 AC bonus).

I was wondering if there was a way to recover the property, or if in the future an option for adamantine armors can be added alongside mythril ones for the NPC special appearance changers?

Best way to proceed is to ask a DM in game. Explain him/her the issue. As the lost property is one attached to the armor material, it should be no problem.

As a sidenote, I would advice you to take a screenshot of the properties of a valuable item before changing its appearance, just in case, because on the server items are not refound if you do not prove you had it; sometimes the app changer "eats" the items, although this is extremely unprobable to happpen. And, in case it happens, another one after, because in the gane window should appear a " You lost ---" or similar message.

Sadly, from what I understand, that DR type doesn't work anyway.

sure it (adamantine) does, but it does not stack with other DR-feats