I definitely have some friends who are published authors and have talked about writing fanfic of their own work -- one of them did so this year for Yuletide under a sock and talked about how it did feel quite different to her than writing an "official" short story set within the same canon. She said mostly that she felt if she's writing canon there has to be plot and explicit character growth, whereas fanfic she just got to play around with them in the existing setting and kind of letting them be comfortable.