Channels using demo plugins such as SimSynth Live, WASP, Beatslicer, SoundFont player, DX-10, ... will be erased when you save your song.

Those demo plugins are there for demonstration purposes only and are not meant to be 'used'.

This way of working is indeed different than in previous Fruityloops versions, but than again, there were other problems of people believing they had those plugins as reals plugins. That's why it was decided to erase the tracks using demo version of the plugins.

To solve your problem you can do a couple of different things :

Don't use these plugins in real productions (they're there for demo purposes only)
Replace the demo-channels by (for instance) dropping a 3xOsc preset on them.
Order these plugins.