Does Yone, Blademaster's ability trigger if you use Void Assault to move him into a battlefield while simultaneously moving the only enemy unit out?

Ruling: Yes, Yone's ability will trigger. When Void Assault moves the enemy unit out and Yone in, the battlefield loses its controlled status during cleanup before the showdown begins, becoming uncontrolled. When Yone then conquers the now-uncontrolled battlefield in the resulting showdown, his ability triggers.

Sequence:

• Void Assault resolves, moving Yone in and the enemy unit out
• The battlefield becomes contested
• During cleanup (before the showdown begins), the opponent loses control per Rule 187.4.c because they have no units there and no showdown is yet ongoing
• The battlefield becomes uncontrolled
• The non-combat showdown opens and Yone conquers the uncontrolled battlefield
• Yone's ability triggers

Nuances:

• The key is that showdowns are only staged (not yet ongoing) during the cleanup phase, so Rule 187.4.c applies to remove control before the showdown actually begins
• This same logic applies if using Void Assault to move Yone into an already uncontrolled battlefield while pulling an enemy unit there