I’ll propose an explanation for why this may be happening. In people who stutter, it’s been shown numerous times that there is reduced blood flow to brocas area, which is responsible for speech production. So by ourselves, brocas area can be activated just fine because the demand on the brain isn’t high. However, in situations that require more demand on the brain, brocas area is unable to receive adequate blood flow that leads to its underactivation. Even at rest, it has been shown that brocas area is underactivated. I can provide references if anybody would like to do some further reading. Personally, I sometimes stutter when alone.