Apparently, if player orders a planetary anomaly mission while the full fuel requirement is still 50 units, the reduced fuel requirement for the planet mission is 35 units.

If Advanced Martian Engines tech then completes research before the mission flies, the full fuel requirement drops to 30 units. The game engine gets confused, can't complete fueling to 35 units, and never completes the mission. The mission just hangs forever trying to fuel up to 35.

You can't cancel the mission and order it again later.

Luckily, attempting to cancel the mission does make the ship available again, and it can be sent back to Earth.

Other missions can be flown later, at the new fuel requirement level, but never the hung mission.

This has happened to me twice. Different game starts.