No problem, Leonardo.
I think you’ve discovered a bug, so I’ve moved your post to that category, if that’s okay. I had only tested this in list format, not board. When I tried it in a board project, I experienced the same problem.
I think the error occurs when the tasked is made into a subtask but remains part of the parent project. In a list project, when a task is made into a subtask, it is no longer attached to the parent project, so it disappears from view when it’s moved. In a board, that doesn’t happen. So you have a task that has been converted into a subtask but is still part of the parent project, which means it shows up in the task list.
@Marie or @Edda, there’s a similar post here, if you’d like to merge them: