Drag one epic with three features and the backlog count falls by four. Nothing went wrong — all four are now placed, and all four will come off again if you remove the parent.
Dragging an epic from Now to Later takes its nested features with it, so a parent and child are never split across boxes by a move. If you want a feature in a different box from its epic, drag the feature out on its own.
Perfectly allowed. A feature can be on the roadmap without its epic; it is drawn as an ordinary card. That is common when only part of an epic is committed to this period.Related: How do I nest a feature under an epic?