When Yi has a defending unit that gets +2 might for being the only defender, and then a hidden unit on the same battlefield is revealed, does the original unit lose the +2 might bonus?

Ruling: Yes, the original unit loses the +2 might bonus when the hidden unit is revealed. Yi's ability is a continuous effect that uses "while", so the moment a unit is no longer the only defender on a battlefield, it loses the might boost.

Nuances:

• This differs from "when" effects which would lock in the bonus at a specific timing
• The continuous "while" condition is checked constantly and updates immediately when the game state changes