Here is that photo of those harmonics I mentioned, Will. Quite interesting -stopped me going off to bed there - looks to me as if the 750 is a 1st harmonic of the 1500 (fundamental f) and then at ~2250 central a first harmonic (wider because of the multiple) & then to the left of 3000 (central) one leg of the 2nd harmonic.

I see what you mean. You have not only the harmonics, but a sub-harmonic too.  I wouldn't know where to begin with answering why this is happening within FLdigi. Hopefully someone else may provide a clue as to why this is. For me with a direct sampling connection from the external speaker socket on the radio, I only every see the fundamental.