In SERVER-50486, we discovered a race where a primary may step down, attempt to call invokeWithSessionCheckedOut in a user thread, and eventually hit a MONGO_UNREACHABLE invariant due to a race.

SERVER-50486 addressed this by setting the setAlwaysInterruptAtStepDownOrUp flag on the opCtx . It seems like this issue is still occurring, we should investigate more into why a user thread was not killed upon step down.