Tasks seem to lose their assignee when they’re changed to a subtask. It seems like they should keep their assignee if the assigned user has rights to whatever project the new parent task is under.
My workflow is usually to create a task, then fill out additional attributes like project, tags, assignee, and other simple things that can be done quickly with keyboard shortcuts. Later, I go through and change tasks to subtasks if there’s an existing parent task. It’s kinda annoying to have assignee undone, since assignee can be done quickly with keyboard shortcuts and changing a task to a subtask is more difficult.