Tested in 28a3ee9.

Spectator is watching as the active player throttles up the ship for a maneuver. The spectator sees the navball throttle setting move but the engine does not activate at all. Fuel consumption on the tank does not go down either. The next protovessel update may have updated the fuel levels for that instant.

This appears to be a regression as this worked in previous versions.

I'm pretty sure there was a situation when protovessel updates were broken, I believe this bug has been fixed for quite some time, although I'm unsure by which commit.