openmw.org

I tried Googling it but couldn't find something about.

My character is equipped with a full Adamantium set and Bonemold helm, and sometimes creatures' attacks gives the has-hit-flesh sound, and my Medium Armor skill isn't trained. Is this intended?

Related code is here.

Are you sure that you have the full set, including shield? Slot_CarriedLeft (shield) has the 10% chance to take damage.
If there is no shield in this case, there is no armor skill training, there is an Unarmored skill training instead.

I had no shield on; that was it. (But yeah how unusual, the requirement for a shield...)