これはXcode 10のKnown Issueでは？

5:20 AM

When build settings are defined in an xcconfig file that inherit values using $(inherited) or $(<setting_name>), the Build Settings editor shows the last assigned value for the setting instead of the properly composed value. (28572636)

5:21 AM

生成されたマニフェストを読むと、ビルド時に"-DRELEASE","-DSTAGING"どちらも渡されてる。