Shimano Steps E6000 battery charge percentage falls abruptly When the battery charge gets to about 20-25% it drops abruptly to 5-10%. The steeper the hill and higher the load drop percentage is higher.
Examples:
from 25% to 6% steep hill
from 13% to 8% small hill
from 19% to 8% steep hill

All in High assist. Bike is 2 weeks old / 250km ODO

Is this normal?
This is a sign of an unbalanced pack. One of the cells is weak and under load its internal resistance is increasing and voltage decreasing. 

Normally this only happens with an old, worn out pack that hasn’t been balanced or has mismatched cells. 

If your pack is new, I’d get it replaced ASAP under warranty.