The web spell is a bit special, in that it initially fills a 20-foot cube, but can collapse and/or disappear if the chosen placement does not provide sufficient anchorage:
You conjure a mass of thick, sticky webbing at a point of your choice within range. The webs fill a 20-foot cube from that point for the duration. The webs are difficult terrain and lightly obscure their area.
If the webs aren't anchored between two solid masses (such as walls or trees) or layered across a floor, wall, or ceiling, the conjured web collapses on itself, and the spell ends at the start of your next turn. Webs layered over a flat surface have a depth of 5 feet.
Notably, a web placed in midair with no contact with either the ground or two solid masses "collapses on itself", but the spell does not end immediately, instead lasting up to 1 round before ending. So, what happens during that 1 round before the spell ends? Does the airborne web simply fall straight down until it lands on the ground, turning a 20-foot square into difficult, restraining terrain for 1 round? Or does the web collapse uselessly to the ground, immediately ceasing to have any mechanical effect and effectively ending the spell as soon as it is cast? If the web falls, is there any possibility that it could catch flying creatures in or under its area as it falls?