I agree with the comments, it can happen. I always stuttered when alone but with practice I now don't stutter when alone. However, if I tell myself that: 'It makes sense to stutter when alone', then I experience that I stutter again when talking to myself alone. Basically, I can switch between stuttering and not-stuttering by changing my mindset. Of course, I practiced a lot to reach this phase where I can switch by approaching the psychological part