Did you enable Z-Levels in the actual world menu, or did you enable one of the mods that adds z-level buildings?
Z-Levels is still extremely experimental and at times unstable, but one thing that should happen is that the world no longer ‘freezes’ when you go downstairs, or vice versa… Without it on you could run into a basement, find twenty zombies and just run out without any of them following you back up… But with Z-Levels enabled they can (and often will) follow you back upstairs because the z-level no longer freezes in place…
One thing this means if you’re NOT using z-levels, you can basically use the basements of houses as permanent food storages that will keep your food from spoiling… Or dump a bed down there and be assured safety from any topside dangers.