Building XWiki platform with trunk in one go produces
The mentioned project properly declares the required build extensions in its POM and builds fine in isolation.
The issue boils down to the active component map DefaultLifecycleExecutor.lifecycleMappings not being properly updated when switching the projects in the reactor. In detail, AbstractComponentCollection.checkUpdate() from Plexus doesn't update the map if the number of component descriptors hasn't changed. This apparently misses the situation where the total count of component descriptors from two different class realms is the same but the set of role hints has changed, e.g. [one, two] vs. [one, three].