Timewalking dungeons give virtually no XP from mobs between 71-80.
At 70 I was getting ~1700xp per kill. At 71 this dropped to ~120 (both with rested XP). Each dungeon you get maybe a sliver of a bar from mobs alone and, no, the dungeon finder bonus is not making up the gap.
Then it got hotfixed because a few days ago it was 22 dungeons 70-80 for me, and since dungeons are like 6 minutes with a competent group it was very fast
I was actually wondering about this. I finally brought an alt in there today, because everyone said the XP was great, and when I was done with the dungeon I had gained less than 1/4th a level in half an hour. So I just went back to questing and got the rest of the level in about the same time.
Definitely need a hotfix for this