This turned out not to be an issue with ConfigurableAction (fields), but in a pex_config ConfigDictField who's owned config itself had a ConfigurableActionStructField in it. ConfigDictFields were not collecting the imports for the configs they owned. This has been fixed, and a test added.

For posterity, since at first I was not totally convinced that ConfigDictField should be responsible for saving its imports; this support was not there originally because the config classes held by ConfigDictField are fixed, not dynamic, and hence I thought maybe this was a new issue created by ConfigurableAction that should be fixed in ConfigurableAction somehow. But I'm pretty sure that's not the case: the same issue would also have arisen if we had ever put a Config class that had a nested dynamic field (like RegistryField or ConfigurableField) in ConfigDictField.