I’d recommend signing apps outside of Kodular, AI2, Thunkable, … (with AppToMarket, APK Editor Studio, etc). So you have no problems with saving, importing and reimporting keystores.
I have been doing this for many years and have never had trouble signing an app if I switched between the AI distributions mentioned above or for whatever reason an app needs to be re-signed. If there is time, I will make a tutorial on this topic.