EDIT: Disregard this... the seventh completion worked. (Despite being the same as previous plates?)

I'm trying to do a progress mission (stage 5 normal sewers, obtain atk breastplate) but it isn't registering. 

I've, since obtaining the mission, gotten 6 breastplates (atk) from stage 5 sewers, on normal, and it still says 0/1. 

I have closed and restarted the game, accessed the level both manually and via clicking on the mission. 

Is there a reason it's not registering as being completed, despite the requirements having been met more than once?