There is no mammoth spawn on Siptah and killed Elephant not count so impossible to complete → lose out of a x10 exp since i got no reroll left for the day.
Tried those mastodon thingies too?
In any case there are some challenges occasionally that are for the other map only
i did, not counting neither
sad 
in that case you can’t do that there unless you go in single player and spawn one or so