When creating a new world the game creates pockets of water that replace the parts of the ground. For the player, it generates normal air blocks however for the detect command they are not air blocks.

Example: I have made a simple function that tests if the block below a tagged armor stand is air and if so teleports it down a block (this is repeated 300 times). However, when the function gets activated it gets stuck on the "invisible" blocks. (Video Linked)
https://youtu.be/1-OruOG155s