Why does adding a Maven dependency breaks the "artifact"

I've created a Maven web project using the Web facet.
When I add a Maven dependency (using Code -> Generated... -> Maven Dependency) the dependency is added but adding the dependency "breaks" the :war exploded artifact.
There's a handy "fix" button on the artiact management dialogue which fixes the problem but this seems very clumsy.
Am I doing something wrong?

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