It all still mostly worked, but only if we scraped our dishes well and if I spent a frustratingly large fraction of my time picking at the sprayer arms with a bent paperclip. Still less work than handwashing, though not by much. I read the manual, forums, etc; couldn't figure it out. Eventually we called the manufacturer and they sent someone out. They pulled the filter, and saw that the macro filter (left) was present but the micro filter (right) just wasn't there.

As soon as the tech pointed out that the filter was missing this very clearly explained what we'd been seeing. I ordered a new one, installed it, and the dishwasher worked perfectly again. What I really don't understand, though, is how the micro filter could have gone missing?