Extremely belated, but...
In the "app.record.index.delete.submit" event handler, since event is not being returned, attempting return false after async/await in the custom JavaScript side for the delete.submit event becomes ineffective.
Please add return event to fix this.
kintone.events.on('app.record.index.delete.submit', (event) => {
RECORDS = RECORDS.filter((record) => {
return record.$id.value !== event.recordId;
});
});