I'm not sure what to suggest, but perhaps the RDL has some sort of glitch in it that is preventing the refresh of the included columns. I suggest deleting and re-creating the dataset based on the stored proc. As long as you recreate it with exactly the same name and all the same properties, everything else should work without altering anything else.

Also, previewing the report will force a refresh on the cache as it executes the stored proc to populate the report, so that might help too. I'd try that first before blowing away and recreating the dataset.