In the “My collection” I found two Anthonidas and decided to dust one. It turned out one card. Now I have 0 Anthonidas. Get by card back!
Did you see 1 Antonidas with a count of 2 under it or did you see 2 different ones ?
Antonidas is currently in the core set so I expect that you saw 2 different ones : the one from the legacy set tha tyou disenchanted and the temporarily free one from the core set
If you filter your collection correctly you should still see the core set one
Antonidas is a current Core card, so it should be impossible to be missing from your collection.
How, exactly, did you search for it? I notice that you misspelled the card name in your post. If you searched for that same name in your collection, then of course it did not find anything, because that is the wrong name.
Also, be aware that Core cards are on temporary loan from Blizzard. They might stay a year or two or three, but eventually they will rotate out of Core. So, because you dusted the only copy that you actually owned, there will come a day where you don’t have that card at all.