https://developer.apple.com/documentation/watchkit/wkextensiondelegate/1650877-handlebackgroundtasks
https://developer.apple.com/documentation/watchkit/wksnapshotrefreshbackgroundtask?language=objc
watchOSのhandleBackgroundTasks で WKSnapshotRefreshBackgroundTask を拾って setTaskCompletedWithDefaultStateRestored しないと watchOSアプリが落ちることがわかった。
If you do not call setTaskCompletedWithDefaultStateRestored:estimatedSnapshotExpiration:userInfo: on the task, the system uses all available time, wasting battery power. The system then suspends the extension as soon as the allotted time has expired.
とあるので、落ちるとは書かれていないが、suspend されることで落ちてしまうのだろうか。