Simply a convenience release, aiming at having the actual next release (1.0.0-RC13) be fine with Java 9. See the release notes for 1.0.0-RC13 for the actual list of changes.
The particularity of 1.0.0-RC12-1 is that its sbt-shading plugin generates code that is fine with Java 9, but it itself doesn't run fine in Java 9. On the other hand, 1.0.0-RC13 is fine in both cases (generates code fine with Java 9, and can itself run with Java 9).