The same thing happens to me.

The rewards are uncommon ICRs. From all over Standard. If you so happen to have most if not all of the Standard legal uncommons completed, then you only get the tiny increase of the vault for excess uncommons. There is no duplictae protection for ICRs.

Edit:
Apparently, Arena also simply does not announce the cards you are getting, when you do get cards. They will show up in your collection marked as "new", with the usual caveat, that that will only show if you have not gone to the deckbuilder or collection already since claiming the rewards. If not, they are added, but unless you have a perfect memory of all the uncommons you were still missing at that time, you can't really determine which one you got.