When placed over air, sand and/or gravel blocks will start falling at normal speed for the first 2(ish) meters, then glitch straight down to the ground (nearest block directly underneath). Found to happen at most distances and all possible circumstances.

For me it was a one time occurrence, able to be reproduced in that particular session, but when I reopened the same level and a different level in order to attempt to capture it in a video, the mechanics worked fine.