These say that they give a plus one AC. However, when I put them on, nothing happens, and an AC Decreased sign appears next to my portrait.

Bug, or do I miss something?

Bracer AC only gives AC when you wear armor with 0 or less base AC.

If you wear anything besides clothing, you don't get the AC.

Most of your questions and bug reports could be answered by reading our official announcements, FYI.