How to reproduce:
1. Open the user's attached "SBP-MovedSceneFile" project
2. In the Menu Bar, click SBP->Build Scenes
3. In the console window, notice that the build was successful, the FileSize is about 20000 bytes
4. In the Project window, move the Scene file to the Moved folder
5. In the Menu Bar, click SBP->Build Scenes

Expected result: In the console window, it says that the Build was successful, the File Size of the Bundle is about 20 kb
Actual result: In the console window, it says that the build was successful, the File Size of the Bundle is about less than 100 b

Reproducible with: 1.11.1(2019.4.14f1), 1.12.0(2020.1.12f1), 1.13.1(2020.2.0b10), 1.14.0(2021.1.0a4)
Can't test: 2018.4(can't run the script, get a bunch of errors)

Note:
- Clearing the cache after moving the file will get rid of the problem, you can do that by clicking SBP->Clear BuildCache
- It seems that the uses the cached path, instead of the path in AssetBundleBuild, I could be mistaken though