Technically there is no bug in error reporting. fsync() reported an error as it should. The application continued processing, instead of stopping. fsync() didn't report the same error a second time, which leads to the app having problems.

The application should have stopped the first time fsync() reported an error. LMDB does this, instead of futile retry attempts that Postgres et al do. Fundamentally, a library should not automatically retry anything - it should surface all errors back to the caller and let the caller decide what to do next. That's just good design.