Ground combat fails with all-fighter loadout

If I put all fighters and no troops into a planet-assault ship, it drops the fighters on the planet. Two things happen:

Further drop ships don't want to add ground troops to the planet. Space combat just times out before the marines can land.

Even though the fighters appear as occupying troops on the planet, the assault sequence only shows the enemy ground troops moving out with no one to engage. None of my forces appear, and so I wait for the thing to time out.