Ah this is what you guys are talking about at Rabbithole xD
I think this happens to other items too.
I’ve found that if you drop the item and pick it back up then check again it will be discovered.
Discovery takes some time (especially because of lag on older servers) AFAIK. I may be wrong though.
I thought trade portal discovery was automatically disabled in 1.5-and-earlier worlds.
For 1.6 and below items only