It logs you out after you save a creation in the Creature/Tribal/Civilization/Space Stages. That doesn't mean you can't login from the settings after this happens though and share that specific generation of your creature.
Long long ago (we're talking like 2009 here). Your creations would be auto-shared to the Spore servers. I suspect they actually changed this due to lack of storage, or 'inappropiate' content being auto-shared. Just my theory though. I did ask a Maxis admin on the Sporum forums (I think back in 2015) to turn the feature back on due to dwindling creation numbers, but I guess that never happened.
Probably not even a bug, my post here from 2014 is most likely incorrect.