Dart build_runner does not work as expected

Question

I've created this small repository where I am displaying this bug. Dose anyone knows what exactly I'm missing or is it a bug?

Steps to reproduce:

1. Clone the repo: git clone https://github.com/Vildnex/reflectable_bug
2. Get the dependencies dart pub get

Now the case where everything works fine is:

1. Run dart run build_runner build
2. Everything works fine and will output this:

[INFO] Generating build script completed, took 452ms
[WARNING] Throwing away cached asset graph because the build phases have changed. This most commonly would happen as a result of adding a new dependency or updating your dependencies.
[INFO] Cleaning up outputs from previous builds. completed, took 3ms
[INFO] Generating build script completed, took 105ms
[WARNING] Invalidated precompiled build script due to missing asset graph.
[INFO] Precompiling build script... completed, took 1.1s
[INFO] Building new asset graph completed, took 1.1s
[INFO] Checking for unexpected pre-existing outputs. completed, took 1ms
[INFO] Running build completed, took 2.1s
[INFO] Caching finalized dependency graph completed, took 30ms
[INFO] Succeeded after 2.2s with 1 outputs (6 actions)

Failure case:

1. Change the name of the lib folder to whatever else, in this case will be main_dir
2. Change in build.yaml the lib folder to the new name, in this case will be main_dir
3. Run dart run build_runner build
4. Nothing is going to happen and the build_runner will output this:

Built build_runner:build_runner.
[INFO] Generating build script completed, took 311ms
[WARNING] Throwing away cached asset graph because the build phases have changed. This most commonly would happen as a result of adding a new dependency or updating your dependencies.
[INFO] Cleaning up outputs from previous builds. completed, took 4ms
[INFO] Generating build script completed, took 56ms
[WARNING] Invalidated precompiled build script due to missing asset graph.
[INFO] Precompiling build script... completed, took 1.1s
[INFO] Building new asset graph completed, took 870ms
[INFO] Checking for unexpected pre-existing outputs. completed, took 1ms
[INFO] Running build completed, took 4ms
[INFO] Caching finalized dependency graph completed, took 30ms
[INFO] Succeeded after 49ms with 0 outputs (0 actions)

Doesn't matter if you change the name of the folder to something else, the build_runner will always output the same thing.