@linuxgurugamer
Sorry to bother you again. After reinstalling some mods I encountered my original problem. With the mods Rational Resources, b9 part switch, and Cryogenic engines. It seems like Smart Stage is having trouble with handling tanks with mod fuel in previous stages. This is causing the same behavior as the previous post.
Steps to reproduce
1. Add a fuel tank
2. Change fuel type to anything not vanilla fuel type
3. Use corresponding engine and add decoupler
4. Copy/paste stage a few times and use smart stage.
Player log: https://drive.google.com/file/d/12svJdMaZ5Hh8_x-3in7i8RZ4ag533u5c/view?usp=sharing
Mod list: https://drive.google.com/file/d/1WXpoIuScy6rgdKQVbqKkE6xp3RiHHAno/view?usp=sharing
Craft showing situation: https://drive.google.com/file/d/19FQe7uD34nXaZeqF3dLvl08KrBVcXJ6x/view?usp=sharing
Thank you again for your work, really appreciate it.