try copying a minecart with pickblock+Ctrl with items inside.
it won't copy the NBT inside...

try copying a brewingstand.
NOW THIS IS REALLY WEIRD
it says (+NBT) but it doesn't contain the brewing items I put inside...

how to reproduce.
put down a brewingstand and put potions inside
now pickblock+CTRL on the brewing stand.
now place the new brewing stand.
no items are inside!