I did.
Made sure the game showed the save-icon and was fully out of combat.
Upon restarting and re-entering the game, the tank was 100% again.
Checked this with my binocs > tech view.
You saw it in the trailer for the game. I have an idea they stopped that fairly soon. I think it was when they started “respawns” which before that I don’t think happened. They will sustain damage in the session, thus you can fast travel way and come back and it’ll still be smoking. You can come back five hours later, provided you haven’t logged out. Once you log out and back in it’s all back to normal.
Shame, I liked the idea of continuity, and if Rivals are continuous, damage could be.