Different Time Formats When Using SelectedFields Parameter

I am querying the web API to get a list of event frames from an asset database (assetdatabases/{webId}/eventframes)

If I query it without the 'selectFields' parameter, the event frame StartTime and EndTime values are returned in the usual ISO format "2019-04-29T19:20:02.75Z"

However, when I specify ?selectedFields=Items.Name;Items.StartTime;Items.EndTime, then the Start and End Time values are returned in this format "30/04/2019 3:21:35 AM"

Is there a reason for this? Or is it a bug and likely to be fixed in future versions?

We're using PI Web API 2018

Thanks