The user look up functionality on a dashboard filter is stripped for any user fields that are in a data design created MDA table.
Normally when you create a report and filter by user ID, GS automatically enables a user look up functionality which is so fantastic and removes any issue of misspelling or needing to capitalize a name (since that can be an issue, depending on where the data is stored).
Unfortunately, that functionality goes away when you create an MDA via data designer. Users then have to manually type our their name or others, which is not the end of the world, but an inconsistent user experience that can be hard to explain