I would suggest doing some testing with creating a new phase and/or deleting this one and adjusting the elements. This may be a file corruption issue or something unrelated to Dynamo specifically. Perhaps create a new file and adjust / add the Phases to test. I have done in in R18.3, and I cannot recreate the issue. If it persists, you should start a new topic.
Phases are read in different ways for various element types. You’ll want to query your phase differently. I believe Clockwork or Archilab has nodes for this (but I may have the package incorrect).