When a unit with Assault that has lethal damage marked on it is moved to a different battlefield, does it die when it loses the attacker designation?

Ruling: Yes, the unit dies. When the unit is moved to a different battlefield, a cleanup occurs that removes the attacker designation. This causes Assault to fall off, and then another cleanup occurs that checks for lethal damage and kills the unit.

Sequence:

• The unit with Assault and lethal damage is moved to a different battlefield
• A cleanup occurs that removes the attacker designation (since it's no longer in active combat)
• Assault falls off the unit
• Another cleanup occurs (triggered by the state change)
• The unit is killed due to having lethal damage without Assault's health bonus

Nuances:

• If using a spell like RTW to move the unit, cleanups may not occur until after the spell fully resolves
• The exact timing of when cleanups are invoked during the move process may involve multiple cleanup checks