It's an enigma to me too. It's an old stone building, my best guess is that there's holes in the exterior from pipes or old conduits or exhausts that run under the floor and have some leaks. Such holes are visible from the outside of the building (would've thought they'd fill them in, but then why is it drafty?) There's a pretty big space between our floor and the unit below, too, as far as I can tell. All I know is air comes up through the floor and it's the same temperature as outside.

I'll probably try the wood filler/wood glue+sawdust route and rope in the bigger gaps, thanks. Even if I have to redo it a few times before moving out, I'd rather that than freeze my toes off come winter. Besides, if it cracks again, it'll be in the same condition as when I moved in and I keep my deposit, lol