"Won this month" is always 0, even right after winning a deal

What I did

Opened a lead that was in the proposal stage (Rui Martins), clicked Mark won, and went back to the Pipeline page.

What I expected

The "Won this month" number to go from 0 to 1. Also, on the lead page, "Closed on" to show today's date, since we just closed it.

What actually happened

The stage changed to "won", so the button did something. But "Closed on" stays empty, and "Won this month" still says 0. I marked a second lead as won just to be sure and it is still 0. It has been 0 since the day we started using the app, which I found strange at the time but only now realised is a bug.