Packs should be fixed to avoid ID collisions regardless, but (if that is the cause) it should nonetheless be fixed. In particular, physics / bug test levels that aren't intended to be played seriously, I wouldn't necesserially expect the same efforts to avoid ID collisions to be made for those (and indeed, I don't always do so myself), but preview differences could still be important to see. I vaguely recall writing code to cache level previews; it's very possible that it identifies the level based on level ID (and if so, it can probably be changed to instead rely on file path).
Beyond that, I'd have to look into it to say if that is indeed the cause (and if not, what is).