This had nothing to do with how the mapping was defined, or any settings in the source (or lookup) data object.
I could write the data to a file, but I could not read that same data back into my INFA mapping.
The cause, in my case, was buried in the Preferences menu of Informatica Developer:
This setting controls how the mapping behaves when it is run directly from Informatica Developer. Changing the setting to “Read all characters” fixed the issue.