I managed to get the sweep icon on a duplicant. I really don't know how, sorry. I may have been selecting a sweep priority at the time. I watched the game for a few minutes and everything was normal otherwise. The dupe didn't disappear into storage. But the sweep icon followed her around. Reloading the save, the icon stayed on her. Save file attached in case that helps.
Steps to Reproduce
I might have selected priority six for the sweep action and highlighted minerals and duplicants together. I couldn't reproduce this though.
This issue has not been confirmed by a developer yet.