I wouldn't be surprised if incremental builds are broken in the gwt-maven-plugin, just like in almost every Maven plugin out there: https://cwiki.apache.org/confluence/display/MAVEN/Incremental+Builds
There's a note about it in the sources: https://github.com/gwt-maven-plugin/gwt-maven-plugin/blob/d6464421d9ceb815db2d594ae0bf2d9ca07a1643/src/main/java/org/codehaus/mojo/gwt/shell/CompileMojo.java#L390
I tend to agree that, the maven-compiler-plugin being broken in a similar way, you should do a clean
anyway in this situation. Alternatively, if you know the change is compatible with your code and shouldn't change the output of the maven-compiler-plugin, then force the GWT compilation by passing -Dgwt.compiler.force
on the command line.