How to delete a record key in Cognito identity pool dataset?

I use Cognito Sync "updateRecords" function in JavaScript SDK v3 (https://docs.aws.amazon.com/AWSJavaScriptSDK/latest/AWS/CognitoSync.html#updateRecords-property) and set "Op" in "RecordPatches" to "remove", but specified record(s) not deleted and "SyncCount" increased by 1.
I also get the same result with same operation in AWS CLI.

What is the right way to delete a record key (not just record value) in dataset ?