For that KLM flight, you will need to check if the flight credited to Flying Blue. If it did, you will first need to have Flying Blue reverse the credit before KLM will tell Delta it's ok to credit the flight: this is something Flying Blue has the reputation for making very difficult. Since it was a business class flight, unless you checked a third bag you have a fairly strong argument that you didn't take advantage of your FB Platinum status (unless this was a flight that wasn't to/from North America, in which case the recent seat selection changes might be relevant...) which might help your case with Flying Blue.

If you used the KLM app after checking in to look at flight status, it honestly would not surprise me if that re-added your FB number: it would be perfectly in character for AFKL to do that.