The cards in your Draft pool did not disappear. The star symbol merely switched to showing only those that you have a cosmetic for. The one card remaining was the only card left in your sideboard that you had a cosmetic for to use, I presume. Click the symbol again to switch back to the normal view for your card pool.

To set your default cosmetics, you can go into your collection and go through all the cards with the double arrow (<>) on them (probably should do this by set, since you will otherwise be occupied for some hours, depending on the size of your collection and the amount of cosmetics you have). Those are the cards you have multiple versions and/or cosmetics for. Click the <> symbol to expand and you can select your favorite version or style for that specific card.

Click the star symbol while deckbuilding, and Arena shows you all the cards you have in your deck with all available versions and cosmetics, and you can then individually select the versions you want to use in that deck.