Reporting version 2019 R3 SP1 (184.108.40.2063)
In the Standalone Designer, with a .trdp file open, you can see the fields from the data source in the Data Explorer. For me, these come from a stored procedure.
If change the stored procedure to add/remove columns, then click the "Refresh" icon at the top of the Data Explorer, nothing happens; Data Explorer still shows the old items. Closing/re-opening the Reporting tool will fresh the Data Explorer list. You should not have to close/re-open. Isn't that what the "Refresh" button is for?