Yes, the flipped sideband definitely indicates need to invert spectrum. External radio mode mapping is NOT what you want! You do need to invert the spectrum on the SDR radio you are using as the panadapter.
Basically it's a two stage process. Find the SDR that you are using in the Definitions panel, then enable the "Invert Spectrum" option for that radio. This then will show the "Invert Spectrum" tick box when you "Start" the radio, which you now need to tick.
You have not said which SDR you are using, but let's assume for illustration that it's the SDR Play RSP2.
So:
1: If the radio is running (Started) in SDRC, STOP it (top left of screen, Home tab).
2: SELECT RADIO button (right next to "Start/Stop" buttons).
3: Click DEFINITIONS (bottom centre of the panel that has come up when you pressed SELECT RADIO)
4: Where it says "SHOW THESE OPTIONS", check "INVERT SPECTRUM" (see picture below).
5: Press SAVE
6: This brings you back to the list of SDRs you have installed (maybe you only have one?) and now the "INVERT SPECTRUM" tick box should be showing. Select the SDR in the list to whch you wish to apply the inversion, tick the INVERT SPECTRUM box, then START your SDR.
7: All should now be well, assuming you put in the correct up/down converter offset for the radio i.e. the IF frequency out of your FT-991, which I am guessing you already did.
Hopefully this will sort it out.
Max
