apparently there's a bug with Copycat and it copying semi-invulnerable moves (Fly, Dig, Bounce, Dive, possibly Sky Drop)
Using Copycat and copying any other move with it will consume PP on Copycat, with those 5 moves however, it doesn't consume PP at all.
Tested this with both Dig and Fly and got the same result both times.
Here's an example of how to replicate this bug:https://gyazo.com/323b99c5c6b2481857c99e3c50b17c58
Give a Toxic Orb, some random move and then one of those 5 moves. I used Fly in my example. Wait all the way until it's only 1 turn left until the poison finishes you off, then use Fly on that turn. Make sure you're slower than your opponent or else it won't work.https://gyazo.com/c4301f3c5010d0024cb6b940df1e173c
Get a Riolu with Copycat and Prankster as ability and a lower speed stat than your opponent.https://gyazo.com/b5313b2e630ad91429ff54b6ce505d4a
Use Copycat with Riolu, you'll go first since you have Prankster.https://gyazo.com/5640d81959c717bf1f61597749549abb
Fly will get copycated if you did everything right.https://gyazo.com/1ba6bd08fa35683688b9a7e4aa80af95
The next turn after Fly was used, look at your moves. Copycat will still have max PP.
Then rinse and repeat. This isn't a display error or anything either. You can just infinitely go for it and it won't ever drain your PP, essentially making winning impossible for your opponent under the right circumstances.
Hope you can fix this soon, because it's a big loophole atm.