I would be interested to read bug reports from people who thought they had a complete enumeration but learned in production that their enumeration was incomplete. I've never seen it myself.
Ideally, of course, the release notes of the library would spell out the issue, and you’d read them, but even then, making sure you fix this everywhere is way easier if the compiler refuses to compile your code if it doesn’t handle the new case.
This type of pattern can be used to force other developers to check if the value is garbage before calling $BUSINESS_LOGIC that uses the enum