When a reporting plugin is used, and that plugin is also present in a pluginManagement section where custom dependencies are declared, those dependencies are not used during site generation.
The issue was originally reported on Stack Overflow. Here's a minimal POM showing the behaviour:
When running mvn clean site, the Checkstyle Plugin, during site generation, will incorrectly use the default Checkstyle 6.11.2, instead of the version 7.5 that was explicitly specified in the dependencies.
A work-around is to add an (empty) plugin declaration in build.plugins.plugin.