I have had a similar experience where the calculation was done at the datasource level and had datasource level formatting, if that is the case what worked for me is creating a duplicate of the field and then changing the default formatting on that duplicate from within the dimensions/measures pane (likely the better option would be to change it at the datasource level but this avoided going through my IT department).
If you created the calculated field within the workbook I am not sure why it would not hold that default formatting as long as you saved the workbook after making the change.
It is very weird, but not sure if it is due to the data being float type or anything else. Anyways, can you go to view and right click and do Format on the measure and see if it does trick or not. Otherwise, if you could post your mock data, we could look at details and see if we can find any solution. Thanks
Default Formating at the measure level didn't work, however, changing the format at the panel level worked. Thanks.