Using Custom Gradle file affects APK build signature?

Been using Cloud Build for years now, and for a particular Android project we've had no issues building, installing builds "on top of the previous one" for updates etc. We also didn't use a custom gradle file.

Now, for reasons related to a plugin update, we are forced to use a custom gradle file. We've edited it to how we believe we need it to be, pushed it to Cloud Build and built successfully. However the APK won't install as it claims there is a "conflicting package" with the same name.

I know from experience that this usually is caused by using same app ID name as another app while its signing credentials are different. However, we have verified and re-verified that the app ID name is correct (in both versions of our app, a "dev" one and a "release" one) and we have not touched the signing credentials configurations set up in Cloud Build.

So... what could be the culprit? We're all out of ideas.

 

Thank you for replying Justin. I'll try to look into that UCB but I doubt we can participate. (Too few resources to risk on experimenting, unless there is compensation involved).

So in the end we worked out that the custom gradle file wasn't being edited correctly by another developer. A dozen tests later we got the app building and installing on top of the old one correctly. (At least so far with the dev builds. We'll be doing another round soon for production.)