Hello, sorry, I forgot about this - thank you for the reminder! I'll start looking into this next.
One thing to note is that currently, copying and pasting sub-cards intentionally doesn't do what you might expect because it's a bit unclear what should happen (should the copied card have a copy of all cards it contained, or should the copied sub-page card point to the same sub-page?). I opted to just make it so copying a sub-page card just copies the card itself, with the contents being blank.