http://www.pcgamer.com/sonic-mania-on-steam-requires-an-internet-connection-to-play/

Users are reporting that the Steam version of Sonic Mania needs an internet connection in order to boot. Those trying to access the game while offline are prompted to connect by a pop up box – though once the game's open, a connection is no longer needed.

Still, this is a pain in the backside, and shouldn't be necessary for a ye olde styled 16 bit platformer with no essential online components. Meanwhile, users are similarly miffed that the game uses Denuvo DRM software, despite the Steam listing previously making no mention of it.