dnd 5e – What exactly happens when I broadcast Web in full flight?

the the Web The spell is a bit special in that it initially fills a 20-foot cube, but can collapse and / or disappear if the chosen location does not provide sufficient anchorage:

You evoke a mass of thick and sticky straps at the place of your choice
within reach of. The canvases fill a cube 20 feet from this point to
duration. The canvases are a difficult terrain and slightly darken their
area.

If the canvases are not anchored between two solid masses (such as walls or
trees) or superimposed on a floor, wall or ceiling, the conjugate strip
collapses on itself and the spell ends at the beginning of your next
tower. The strips superimposed on a flat surface have a depth of 5 feet.

In particular, a canvas placed in the air without contact with the ground or two solid masses "collapses", but the spell does not do not finish immediately, but at most 1 round before finishing. So, what happens during this turn before the end of the spell? Does the airborne slick simply fall to the point where it lands on the ground, turning a 20-foot square into a tough, binding terrain in one round? Or does the Web cease to have a mechanical effect as soon as it collapses, effectively ending the spell as soon as it is launched? If the canvas falls, is there a possibility that she can catch flying creatures in or under her area while she falls?