I went through exactly this. Officer told my fiance everything looks good and as soon as we get medical, youll get visa. Got home and boom ds5535 email. Fiance is Canadian born, no marriages, no kids, no criminal history, etc.
It took me a while to accept that I had to file a WoM. Tried countly emails and got congressman involved. I appreciate that lawyers have something to help, but the cost of a WoM should be half. Its just so much money. I understand love should have no price, but a person shouldnt go broke for it.