Ash of one form or another can be found on many kinds of soap. Some is soda ash (sodium carbonate), some ash is made up of soap crystals, etc. If ash hasn't happened on a particular type of soap you make, that doesn't mean it won't ever happen. Just that it hasn't happened so far. Some things that seem to contribute to sodium carbonate ash --
Pouring soap batter into the mold at emulsion or very thin trace.
Batter isn't well mixed enough so the water content varies, even slightly, throughout the soap.
Recipe is higher in water -- I'd say lye concentrations of 30% or less make ash more likely.
Soap doesn't get warm enough to go into gel.