Power Query Editor custom column correct, but not when apply to model

Hi

I have created a custom column in Power Query Editor below which is working perfectly and returning the values that I want.

= Table.AddColumn(#"Changed Type1", "Duration", each if [End Time] > [Start Time]
then [End Time] - [Start Time]
else ([End Time] - Time.FromText ("00:00:00")) + (Time.FromText ("23:59:00") - [Start Time]))

When I close and upload the values in the model table aren't correct and I can't even really understand where they are coming from.  They are not all the same but large number of rows are returning the same value.

Any help would be appreciated.

Thanks