Huh. As an avid home coffee roaster, this is interesting to learn. I find that decaf also really struggles to “crack” when roasting, and emits way less smoke. I guess that’s because there’s perhaps nothing left to really crack anymore?
Less-so that it has expanded from the decaf process, and more-so that the additional porosity leads to the 'crack' you get from reaching that critical temperature is much less violent, since there's essentially already many micro-fractures in the bean. Think of it like attempting to burst a pipe with a leak in it, vs. a pipe that is sealed. The leak will bleed off pressure, so you need much more flow (in coffee roasting, this would be power from heat) to get the same build up and explosion.
This makes sense! The coffee has expanded a lot from the decaf process so it's not going to expand as much during roasting, hence no "crack" (which is really the same kind of process as popcorn popping)! The reduced smoke may be due to the removal of the skins and residual dried fruit which would have been washed away along with the caffeine, whereas I would expect a natural process coffee to produce a lot more smoke (compared to the most common washed process coffee).