I have found that it occasionally hiccups in offline mode. I play mine allot on my layovers at work (railway conductor/gaurd depending on what side of the pond you live on) and the wifi on the train is unreliable. I find that if I try to start planet coaster in offline mode, about 1 out of 5 tries I get an error saying steam can not connect, check your internet connection or contact your system admin, than try again.
I get this message when trying to launch planet coaster from the desktop. Only solution I have found is to use my phone as a hotspot, let it talk to stem, than once Denuvo? has verified it as ok, I can put stem in off line mode and play the game.