It's really strange that Sega has been hush hush on Sonic Mania and Sonic Forces sales. I think they were holding off on Mania sales because they wanted to see if Forces would do better so they could cement that "We can still outdo our fans", but I honestly think Mania destroyed forces in weekly comparative sales and they don't want to let what is essentially a fan game overtake their own work.
If my speculation is somewhat true, it has me shaky on if there'll even be a Sonic Mania 2 made by the same team as the first game. They'll probably take their engine and use it for themselves. Ugh.