I use BTRFS on my / partition, BTRFS is encrypted with LUKS. My swap is a swapfile inside a dedicated subvolume I made to store it. For hibernetion I use GUI in KDE, so for example I push a button "hibernate" and the computer behaves as if it hibernates. After I resume the computer from hibernation I check up how much swap space is occupied. When I launch system monitor to check up available space in my swap I see that swap is totally empty. It had 0 bytes occupied out of total swapfile's size.

Is this the expected behavior for swap? On my previous Linux distribution swap space always had some leftover occupied space in it, though this was the case when my system ran out of RAM and it had to use swap for the overall stability of the system's operation. In other words some occupied space in swap was present in case when I didn't use hibernation.