To reproduce:

1. In a new project, in the Player Settings for Android, check on Split APKs by architecture
2. Build the project for Android
3. After the build finishes, go to {path_to_project}\Temp\gradleOut\build\outputs\apk\release and open the output.json file
4. Observe that VersionCode for x86 is 300001 and VersionCode for armeabi-v7a is 100001 instead of just 1

Expected: 30000 and 10000 are not prepended to the version code when splitting the build architecture

Reproduced in 2018.2.0f2, 2018.3.0a5
Did not test in 2018.1 and earlier versions, the ability to split APKs by acrhitecture was introduced in 2018.2

By design: each APK must have unique version code.