https://www.youtube.com/watch?v=OLy2wD566kA

RECAP:

Mania Sonic is actually "from another dimension" as the script for Forces says, but it's not because Mania is its own self-contained universe, rather because Forces is its own self-contained universe.

The end of Mania has Classic Sonic is warped to a new universe created on the spot in the image of the one who last touched the Phantom Ruby; Classic Eggman. Thus, a universe where Eggman conquers the world and has defeated Sonic the Hedgehog.

This in turn explains any unanswered questions and inconsistencies with past Modern games, because the Classic characters only saw small glimpses of their future in Generations.

They saw Silver, but they didn't know he was a time traveler, which is why his presence in Forces goes unexplained.

They saw Chaos, but they didn't really know how strong he was in his base form, which is why he's a joke in Forces.

Humans don't exist because the only human in the Classic games is Eggman.

Tails is still a pussy because in the Classic cast's eyes, he's still never experienced his story arc in Adventure where he learns to be more brave and independent. This, along with the subconscious idea that Tails believes Classic Sonic's departure will mean the erasure of his universe, is why Tails is so obsessed with Classic Sonic.

Green Hill and Chemical Plant being so prominent has to do with those areas being so prominent in the Classic universe, while every other major location in Forces doesn't really have any substantial worldbuilding because they're only vague visions of a future that the Classic characters don't fully understand.

Null Space being so easy to enter and easy to exit is because the universe was only just created and is still in an infantile state, only existing within the story of Forces and nothing more.

The story as a whole being inconsistent with itself, counting the prequel comics and Episode Shadow, is because the Phantom Ruby is having a hard time keeping up with itself since it has to create this new universe on the spot with, again, the main idea of it really only coming from the Classic characters' vague memories of a future they haven't directly experienced.

Infinite being loyal to Eggman despite lines hinting that he may turn on him has to do with the idea that the Phantom Ruby itself is still creating everything based on Eggman's wishes specifically, which is also why Eggman himself eventually becomes the final boss of the game, because the Phantom Ruby still subconsciously draws itself to Eggman.

This all goes on a meta level as well because the Forces universe is not only created based on Classic Sonic's memories, but also on the player's memories beyond the fourth wall, hence the creation of the Avatar and the copy-paste NPC's (and any discrepencies not present in Generations, like Zavok, Omega, Orbot and Cubot).

And at the end of it all, Classic Sonic leaves the universe and returns to the main timeline, but rather than the Forces universe being erased like Tails feared, it's allowed to thrive as its own separate world; that being the IDW universe which follows directly thereafter, as IDW is confirmed to be separate from the games.

TL;DR, Mania isn't the self-contained dimension, Forces is; having been created based on Classic Eggman/Sonic's memories of Generations by the Phantom Ruby. Any and all inconsistencies can be explained as the Classic characters not having full knowledge of what happens in the future past 3&K. This self-contained universe eventually becomes the same self-contained universe as the IDW comics.

Oooooooooor Forces is just poorly-written. But in the words of one David the Lurker, "that's no fun".