You can keep that from happening, with these strategies:
Identify not just individuals but work groups. Assign them challenging tasks, and then keep an eye out for the leaders who emerge.
Challenge promising people with assignments to plan future activities, not necessarily to carry them out. It’s a great, low-risk way to stretch people as you assess their abilities.
Encourage people to participate in cross-departmental initiatives. That will get them out from under managers who may be stifling them.
Keep your door open so that promising leaders can use their own initiative to make their presence known to you.