Maybe its about many pokemons. Today sevipers 120~ lvls deal to my pokemon DMG 10k or sometimes more. Not sure I am mistaken or not but saw that high DMG.
Check server log damage to be 100% sure the damage is bugged.
Like Khromes said, sometimes multiple damage instances stack and appear like -10k on the screen. Using the server log you can see how much damage was dealt by each hit.
he's right, it' not a visual bug. pulling atks like fiery tornado and water tornado some times do 4x and even 5x hits, just like lock on and fairy lock (with fewer hits obviously). not sure if it's linked to stacked pokes tho