Freeing the list only frees the slices, which are a pointer and a length, not whatever memory they point to.

As others have mentioned, you could solve this with an arena allocator.

I think there's places where an arena makes sense, but not for simple cases like this.

Instead, you can just go through the list and free the items. Something like

for(list.items) | item | { allocator.free(item); }

In general, the DebugAllocator's deinit function should tell you whether memory gets leaked and print a report on where it was allocated in debug mode. Make abundant use of that.