They can't reach 20000 ft or 6000 metres.
Some can fly at 6000 metres, but other models have service ceilings of 4000 metres.
And if you are thinking of flying from low levels to those altitudes, that's just asking for trouble.
...every time I read a post like this: I remind myself to strap the battery into my Mavic 2's... better safe than sorry. Zip tie, or velcro strap: it'll keep the battery connected.