Was this a GM dealership with GM diagnostic equipment that it was taken to? If they couldn't figure out what the issue was, either they don't have the right tools, or they are suffering a rectal compaction due to cranial insertion. Either way, it'* concerning, unless they weren't able to track it down in the allotted for a diagnostic, and it really is a major gremlin.
If it is a security related issue, I am thinking the original key should work. You can try the security relearn, which I believe involves a series of prolonged key on stretches (interweb search would be good), but if that is the case, I would have expected the blank to keep working.