When creating a card in a Custom Collection, unless it matches the filters of the current collection it might seem to disappear after you've finished it. It's not gone don't worry, it's just no longer applicable to that current collection.

You will be able to find that card again in your Home. Either click Home in the sidebar or press "Cmd / Ctrl" + "Shift" + "H" on your keyboard to immediately go Home.

We've added a little helpful yellow indicator when authoring cards in Collections to remind you that this might happen :)