executing migration via cron vs ui fails with 'missing dependencies'

When setting up a migration with dependencies, I can execute it via the UI. AFAICT it executes the dependencies first and then the main migration script may use migration lookup to grab the dependencies which I believe is correct.

e.g.

migration_dependencies:
  required:
    - twodareis2do_rss_skysports_picture

However when executing via hook_cron, using the above it will complain of missing dependencies. It seems that on occasion I can substitute required with optional, however it would seem that that this seems slightly erratic and will only seems to update sometimes.

Would it be possible to fix this so that cron migration scripts behave as they would to when executed via the UI.