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It seems the Substring widget will not work for Dropdown list field.
It does work on Textbox field as intended though.
Here is a test video
What you can do for a workaround is to copy first the value of the Dropdown list to a Textbox using Condition logic, and the Subsring the Textbox which contains the Dropdown selected value.
Hope this help. Let us know if you need further assistance.
Here is an upgraded version of the test form https://www.jotform.com/61080745371958
I include the workaround solution I mentioned