Now it's tip&trick but it's also a bug. Beholder is flying and more importantly hovering aberration. It should not fall off the cliff.
Interestingly no one really know how Beholder hovers. It's not a magic hovering and cannot be suppressed by antimagic field. According to Monster Manual for 3.5e:
A beholder’s body is naturally buoyant. This buoyancy allows it to fly at a speed of 20 feet. This buoyancy also grants it a permanent feather fall effect (as the spell) with personal range. Monster Manual for 5e doesn't mention any details on this matter.
EDIT: I discovered it by
opening the Iron Flask in cave near the hole. My invisible wizard have 100% to shove it.