Well, I tried with a brand new project and I am also not able to reproduce it with that project.
But by adding Firebase and Google Play Service in the Manifest incl. the JSONs I am getting the same Errors
The twinny project reacts different. As written each of Conditional Compilation working well if I close the IDE after a compilation. But when I have done a compilation and change then the Conditional Compilation I am getting that error.
It doesn't matter which one was compiled before but if I change back to the Conditional with which the last compilation was done it throws no error.
The package name will be changed and the Firebase Ads will be removed from the Manifest, Only Crashlytics remain.
So, with a closing and reopening of the IDE I can work as a workaround.