-- This query does *not* work--
SELECT * FROM templates
WHERE
  FolderID IN (
      SELECT FolderID FROM TemplateFolders
      WHERE FolderName = :Folder)
  AND
  Name = :TemplateName;

If I put in the template name directly in the query then there is no
problem:
-- This query does work--
SELECT * FROM templates
WHERE
  FolderID IN (
      SELECT FolderID FROM TemplateFolders
      WHERE FolderName = :Folder)
  AND
  Name = "MyTemplateName";

I fixed the problem by swapping the order:
-- This query does work--
SELECT * FROM templates
WHERE
  Name = :TemplateName AND
    FolderID IN (
      SELECT FolderID FROM TemplateFolders
      WHERE FolderName = :Folder);