We bought the kit FVR 100 and built it ourselves and used the insulation set from you. The furnace keeps the heat well. I see that the two parts of the dome are about 1-2 cm apart and you can see the ceramic fiber in some places.

The gaps on the dome furnaces may become somewhat larger or wider over time, but they are harmless. Due to the applied insulation of the dome, the escape of the temperature and also the escape of the flue gases is not possible. The material expands at high temperatures and then contracts. It may be that the dome was not held 100% on the outside of the sides and the gap was caused by this. However, I assume that this will not increase in size in the next few years. Further spackling in the dome (inside) is not necessary and would also loosen over time. You can continue to use the furnace as before without any worries.