I am not a 996 expert by any means but here are my thoughts. First, I can't think of anything in a 30k mile service that would impact the cooling system unless they found you to be low on antifreeze and added a bit.
Second, this time of year I would guess your daily temps are in the mid-60's. So I would seriously doubt the fluctuation is normal, maybe in the heat of the summer, but not now. Perhaps you picked up some trash on one or both radiators? There is a great
DIY video on how to clean them.
Otherwise, I would take it back to the shop for further analysis.