Just did a quick test. Started a new character, severed his arm, spawned a Doctor and an Ensistic Arm. Was able to replace the stump with the new limb. Strength went up from 3 (After the penalty of losing my arm) to 8. It probably should be more clear what the item was that you replaced your arm with in the stats/inventory screen.
EDIT: Did you have another arm attached before? I think I found the issue caused by severing the same limb multiple times and reattaching it with non-organic parts.
EDIT 2: It was a bug for sure. Fixed it and made some adjustments to the system. 0.6.1 will have these changes, but I'll put out an experimental tonight. Thanks for the report! :D