I have encountered this bug several times, with default postview messages, on levels that either:
- Have a save requirement of zero. Probably not critical, no real-world level will have this requirement, only placeholder ones. Correct behaviour here is undefined, as both "saved none" and "met requirement exactly" are fulfilled, and in a real-world scenario these are mutually exclusive so no priority between them is defined.
- Have exactly one lemming (including cloners) and require exactly one lemming saved. Correct behaviour here would be to display the "saved all" message.
There may be other situations, but these are the ones I recall. I don't personally remember seeing it on levels with more than 1 lemming, as long as the postview message conditions themself aren't faulty.