Holy cow, did this thread blow up.
I went back to the LDS and mentioned that I appeared to have gotten a low fill, and asked if they would top it off for me. They grabbed a pressure checker and confirmed, yep, totally a low fill in this tank, we'll fix it for you right away. Super gracious and apologetic, topped off quickly, and they even offered a free fill in the future for having to bring it back in. Their best guess is that some of the new people working at the center had only filled LP steel tanks before, and hadn't realized that mine was the high pressure 3442 instead of the low pressure 2400. If that is the case, hopefully they'll make up a sign or remind people to check the pressure stamp before filling.
All in all, resolved to my satisfaction, and I'll continue to patronize the shop in the future. I'll just remember to bring my SPG with me next time and check before leaving. Thanks everybody!