Gradle 6.0 removes source set dependency

I have a collection of lessons for students here:

The dependant lib for this project gets injected via a gradle plugin here:

It all works great up until gradle 5.6.4. But when I upgrade the wrapper, it breaks and loses the Source Set of the above mentioned library. Can anyone help me with that?

Steps to reproduce:

1. Clone repo
2. Use gradle wrapper version 5.6.4 -> Example works. (Source set dependency engineEmi is integrated)
3. Use gradle wrapper version 6.0 or above -> Example breaks (Source set dependency engineEmi is missing)

I added some screenshots for better elaboration