In this SSRS tutorial I will show you how to preserve formatting from SSRS report when you export to Excel. For the purpose of this tutorial I used SSRS 2008 R2.
When you try to export your SSRS report to Excel you might lose formatting. This may happen when you format your data for instance using expressions. To retain formatting when you export to excel go to cell property and change the formatting there.
This simple change allows Excel to understand what formatting you would like to get after the export and hopefully that will fix your formatting issue.
Hope that helps!