SSRS 2208 Multi-value Default Parameters Not Being Used

I spent about an hour trying to track down what was causing my default multi-values from appearing in Business Intelligence Development Studio (BIDS) AKA Visual Studio 2008 (VS2008).

My dataset is simply a one-dimensional list of names of human beings, such as John Doe. The default values are were a subset of those names but they were not being selected either in Preview in BIDS or on the server.

This post gave me the idea to check my list of default values against the dataset. I knew that I didn't have NULL values in my dataset, but I realized that I did have some default values that didn't correspond to the list of values in my dataset. The default values had until recently matched their counterparts in the dataset, but staff turnover had changed two of the values from "Firstname Lastname" to "Firstname Lastname (DOMAIN\USERNAME)". As soon as I removed the two non-corresponding values all was well- the default values were selected as desired and the report again started executing as expected.