not gaining exp in campaign..

I created a new character and rushed through the campaign so I could get to endgame quickly and catch up to my main.

But by the later part of Act 3, I was still stuck at level 20, gaining zero XP from mobs.

The XP penalty system feels backwards, it punishes you for fighting higher-level enemies, when it should only apply to enemies that are lower level than you.

So now what?
Am I supposed to:

Go back to Act 1 and grind zones there

Then repeat in Act 2,

Then again in Act 3...
Just so I can finally start gaining XP in Act 4, the interludes, or later in endgame?

Running the campaign again is fine, but don’t punish players who want to play the game their own way.
Respect players time by removing or adjusting these penalties.