I just fought Nacho Muerte for the new achievement, and I got the usual prizes and exp for 4 out of 6 of my wins, but my 2nd and 4th win just displayed this:

No prize, no experience, but I clearly won the battle.

[edit]

Update 4/7/2018: I just got my 5th win on the five-headed Panzer Hydra, and it didn't display my experience or prize drops. Just the same "You have defeated [opponent]!" message like in the screenshot above. I checked my inventory though, and I did receive the 5th win loot (Panzer Hydra Care). So that's good.

I don't know if I got the experience I should have received, because that's harder to watch for.

I've had this happen too, with one of the smaller event challengers. Edit: I wasn't sure if I'd received the loot/exp or not.

This just happened again with a different opponent, so I updated the thread title to better reflect the issue and added my most recent experience to the first post.

I'm going to ping you here about this.

I definitely didn't receive the loot on the ones I showed you (but did give you the source code on the most recent one - I was watching my inventory, so if the drops are just regular junk, I don't think they're granting it). None of my issues happened on one of the major achievement ones though (5, 50, 75, 100 etc). I'm not sure whether I'd have received loot on the 5 drop because it didn't happen on those ones.

[edit] Mine is reported here in case that helps.