sand doesn't fall standing on a frog spawn

Sand and gravel doesn't fall standing on a frog spawn.

Я ожидал, что произойдет...:
The sand or gravel had to fall.

На самом деле произошло следующее...:
But the sand or gravel did not fall, but stood on a frog spawn.

Шаги для воспроизведения:
1. Place the frog spawn.
2. Place sand or gravel on the frog spawn.
3. Sand or gravel is standing on the frog spawn.