The attached report is built against SampleData and demostrates the issue. The first parameter (IWantAscending) is a boolean parameter that "controls" the second parameter (OrderParam). OrderParam has a post-processing formula that interrogates IWantAscending. If IWantAscending is true, then the value of OrderParam changes to ASC; otherwise its DESC.
The OrderParam value definitely changes, because I can see it clearly change the ORDER BY clause in the query driving the report, and you can see the data gets sorted on change of the IWantAscending parameter. But the OrderParam combobox value never changes, regardless of the OrderParam value being set.
1. Deploy the attached report.
2. Select true for IWantAscending. Note that the OrderParam combobox displays DESC, but the report sorts ASC.
3. Select false for IWantAscending. Note that the OrderParam combobox displays DESC, ad the report sorts DESC.