Multi-module projects rely on JAR packages instead of source code issues?

Since the multi-module project, the modules depend on each other, it is strange to find that they depend on each other to hit the jar package instead of the source code. Find a half-day, finally found the reason. I used the uppercase letters when I defined the GroupID of the project. But when I add dependencies to the POM, I use the groupid as lowercase letters, which is why. The problem is resolved by changing the dependent jar directly to the correct groupid.

From for notes (Wiz)

Multi-module projects rely on JAR packages instead of source code issues?