If you summon an armor stand with gravity turned off:

/summon ArmorStand ~ ~ ~

And then turn on its gravity with ArmorStand#getGravity, the armor stand does not stop falling when it hits the ground. Instead, it falls through all of the blocks in the world and into the void.

This does not happen if you spawn an armor stand with gravity on, toggle it off and back on again. It only seems to happen if the armor stand was spawned with gravity already turned off.

I am not sure if this bug is caused by cb/spigot code or vanilla code since there is no way of changing the gravity of an armor stand in vanilla after it is already spawned.