Update: The mechanic at the shop said he wasn't sure what the deal was. He said he'd seen this exact issue on a couple of their rental bikes last year with 105 RDs and admitted they "just put up with it" but acknowledged that wasn't an acceptable explanation for me. He told me he was going to get in touch with their Shimano rep. and told me to call back in a day or two to see what he was able to find out. It's almost as if the spring has sprung and isn't keeping the derailleur in the proper position.