Thank you, Andrei M., for taking the time to share this!
Apparently the LT-12 mission is bugged (for some players at least). You have to complete it with honours in order to actually pass to the next mission. For example, Andrei M. did more than 1000 damage from passive scouting (actually you have to do just over 500), yet it was not completed.
Official mission requirements (from tanks.gg):
LT-12: The Ghost Tier IV – X
Good concealment increases your chances of survival in battle. Light tanks have great concealment abilities, thanks to their relatively low silhouettes. Main Objective
Luckily, there is a quick fix for that, you have to cancel the mission then start it again and the game will mark it as completed.