When I run mvn generate-sources on a POM which binds the goal purge-local-repository to that phase, then MDEP correctly purges and re-resolves the all dependencies of all submodules.
When I instead run mvn dependency:purge-local-repository then the reactor skips all submodules.
This in rather annoying and surprising, because nowhere in the public description of MDEP is told that invoking dependency:purge-local-repository at the command line as the sole goal will effectively skip submodules of the POM.
If this is not intended, this bug should get fixed.
If this is intended, then the documentation of this goal should clearly tell that.