I kind of depends on what perspective you are concerned with, functionality or principle.
From a functional point of view, personally if it was my rig I wouldn't worry about it one bit. If it was the tow vehicle I wouldn't accept it, but it's just a trailer, and even more remotely it's the spar tire of the trailer. Yes some of us like to treat our trailers like they are precision manufactured performance machines. But in reality they just are not. Every day I see worker trailers with old and mismatched tires and they are going down the road just fine. Even if the size is a bit different the equalizer or the torsion axle will just flex the half of an inch necessary to take up the difference.
Now if you want to look at this as a matter of principle, you ordered 5 tires and they sold one to somebody else. On the principle alone I would be bothered by that. On the positive side you got a free tire out of this so you saved a bit of money. If they are willing to replace the spare with the appropriately matched spare for free I would say go for it and insist. But if they were going to charge you for the appropriate tire I would say leave it alone.
