campaign exp intentionally nerfed or bug?

I just reassembled a Kitava blacksmith. I of course gave him very good equipment from the start. I didn't really pay attention to the level of the cards when I advanced, I just rushed the essential quests and advanced. I found myself at level 27 in act 3 "in the past" map lvl 43. At first I thought I rushed too quickly. Then I started to pay attention and there... when I kill normal monsters I don't see the exp increase... I did a test on a map act 2 level 27 and there I see my xp increase visibly. So I did the following tests at level 27 on normal monsters:

Map level 27: 1200 XP
Map level 28: 1400 XP
Map level 30: 1800 XP
Map level 33: 2200 XP
Map level 34: 2500 XP
Map level 35: 1500 XP
Map level 38: 900 XP
Map level 43: 80 XP

That's when I realized... it's not that I leveled up too fast, but since the XP decreases from the 8th level above mine, I have to kill 15 times more monsters at level 43 than at level 27 to get the same XP. But... why? I don't understand the logic. I'm playing alone, so this nerf is inexplicable.

IDK if it's still a thing, but I seem to remember the sweet spot for monster level being +/- 4 levels from your own.

But what is the purpose of this punitive mechanism?