My release shows no unhandled errors in Issues, but the crash-free rate is below 100% (or crashed sessions are above 0%).

This is expected. Crash-free rate comes from session status, not from Issues.

SDKs can mark a session as crashed even when the matching error event never appears in Sentry. Error events can be dropped by sampling, inbound filters, beforeSend or other event processors, or quota and rate limits. Release Health sessions are not subject to those same filters and sampling rules.

Docs note that crash counts are not expected to match crashed session counts for this reason. See Release Health and Sessions vs. Issues.