Lots of our provisioning handlers have .sig files that we use for security purposes. SDClientHandler doesn't have one. This is simply a red herring currently in the product currently that we expect to see. To further troubleshoot this, I would take a look at the SdclientHandler.log file or the SDCLIENT log file generated from the package and see if you can locate any errors in those logs.
Thanks for the response Aaron! I logged a ticket with support, and the tech helping me with this issue mentioned the same thing about the missing SDClientHandler.sig being expected behavior and not a concern. I tried digging through some of the other logs, but couldn't spot the issue with my untrained eyes, so I'm hoping the LD support person can narrow it down. I'll be sure to update this thread with the resolution/workaround.