We have made some updates to the way DET query files are generated so I don't know if the issue is that you are self-hosted. I will flag this to make sure that it's correct in the query file. Are there any other properties that you would want included in this query file by default?
Using date_closed is correct. For legacy reasons the case API remaps some case properties to different names. If you're in doubt you can always query the Case API and look at the JSON data that's returned. This is the same data that the export tool is using.
Thanks both for the tips. Querying the Case API straight forward so I'll be using this going forward.
I don't think there is a need to add additional fields by default since the user can decide what to add the case export already, and I feel it's better to keep the exported data to what is needed only.