The bottom of the world has many issues and this is a known problem, discussed here:
https://github.com/minetest/minetest/issues/2702To my knowledge, if you dig gravel at the bottom, it will fall through the bottom for about 16 further nodes BELOW the bottom, but it won't fall any deeper. As far I know, in this state the gravel is still “falling”, it won't even stack there, but the weird “endlessly falling gravel” will get deleted after 15 minutes or so on default settings. Of course, this stuff is not good and clearly not intended.
But the gravel will not always fall right through, sometimes it is stopped. The bottom is really weird and buggy (see the issue discussion for for info).
I have created a mod which generates a indestructible bedrock layer at the bottom of the world, so this should be a nice workaround for those world bottom issues:
viewtopic.php?f=9&t=11271