Armor durability won't go down. Damage a player and it stays full always.
If the issue is not reproducible without plugins:
Please use gist or anything else to add other files and add links here
...
This is not implemented yet.
Oh, I thought it has done that before.
It did until this commit
https://github.com/pmmp/PocketMine-MP/commit/efca9f0450fed2c660ca450718670ca71b2c0d82#diff-be0be6071c2c2f0abecaa7dd4e16e2f1
No, it has never been implemented. @95CivicSi that commit has no bearing on armor whatsoever, unless you used a plugin to make it work. It's still yet to be implemented into the core code, as seen here (look at the documentation) and in subclasses overriding that method.
(P.S. Until recently armor did not even work for anything except PvP. The implementation is incomplete.)