That is strange...
Did the quest log get updated saying you completed that quest?
If you talk to Swen again can you go through the dialog options again, and see if that triggers the update?
When you re-tried that part, were you reloading a save before killing the bugs, or before speaking to Swen?
Maybe try equipping the Silver Breastplate, rather than leave it in your inventory (or put it in your inventory if you had it equipped).
Welcome to the forum.
