I bought 3 of these and it looks like there is a small gap at the bottom that exposes the insulation. Is that a safety concern or normal?

That small gap that allows some of the insulation to be visible is normal and is intentional. The outlet or male end of the chimney is fully connected from inner liner to coupler to outer jacket. However we need the inner liner & outer jacket to be able to move separately due to different rates of thermal expansion. The inner liner will see a lot more heat & will want to "grow" more than the cooler outer jacket. So, on the other end, (the inlet or female end) we have it so they are not fully connected. This allows the inner liner to grow without being restricted or held in place by the outer jacket. The gap is definitely not a safety concern, we need it to be there for smooth operation during heat up & cool down.