The issue is discussed here: http://www.openradar.me/33005768
Installer creates .localized directory for application bundle
When installing an upgrade package that contains an application bundle that does not match the CFBundleIdentifier of the application that is being upgraded, macOS installer creates a .localized directory and installs the upgrade in this directory instead of upgrading the application in place.
The solution is to make sure to use the same Identifier between applications