Steps to reproduce:
1. start the scenario.
2. build some belts, underground belts.
3. start the level
4. press back to building
5. inventory stacks have reset to start values.
See this image where i repeated this multiple times and was able to fill the area with belts. Defeats the point of the scenario if you dont have to care about belt limit.
*if there was no intention of having a limit then i would suggest upping the start values to something higher so that its obvious it does not matter*