So with mdf I tend to find that the edge is kind of shiny and carmelized. Not all mdf Is created equal, so yours might have more actual char, meaning like loose carbonized ash.
If that’s the case the salt might help knock the loose ash and carbonized material off.
There is a third possibility too. If you overpower your cuts, especially if they’re slower than they need to be, you can end up with more char than you might otherwise get. I’d suggest doing a material test at full power to back into the fastest you can go to get reliable cuts, and again the same test but with 2 passes. You might find the edge quality changes so that you like it better.
If you’ve never tried it, I have a test method that I like, check out #6: