Sure the 20mm round represents an amazing amount of kinetic energy, but the drone doesn't offer any resistance, and I wouldn't expect the round to even notice. Sure the disturbance in the air might throw the drone for a few loops, but any decent software will recover.
Sure hitting more substantial targets the target itself becomes of the shrapnel that causes an ever increasing zone of damage. Not clear to me that a piece of paper or thin fabric would exhibit the same behavior.
1. I think you're conflating "cheap agile drone" with "foam or balsa+durakote fixed wing aircraft".
2. Probable engagement aspect for a drone is not "straight up", as would be necessary for your scenario.
3. A little itty bitty hole in a wing does a whole lot more than you'd think.
4. Expecting a high velocity cannon round to just punch a clean hole in the wing is a little much.
Foam's a bit more robust, but a small pebble at a few miles per hour has wrecked my whole durakote day before.
And yes, there's degenerate cases, like you could have a whole sheet of dollar tree foam board maybe take a hit or two.
Explosive (self-destructing) and incendiary
https://www.gd-ots.com/munitions/medium-caliber-ammunition/2...
I guarantee you that just grazing a drone with a 20mm round will still kill it, even if you don't "hit" anything critical. :P