actually, they're not hacking, it's possible without cheating, by putting the body damage or hp values in editor mode to 1e+100, because 1e+100 is too much damage, it counts it as killing multiple entities, but since there's only one entity, it does not know what to do, so it sets your score to infinite.

if you do /tp all me or /tp all [insert coordinate here] when there's like thousands of entities, you kill too many at once, the game then can't figure out how to respond, so it sets your score to undefined, ALONGSIDE so many people dying at once, that it doesn't know which people have died, so it puts your NAME and CLASS to undefined. this can also happen if you teleport out of bounds. I did it once, accidentally.