By skyforbes I didn't realize it, but apparently this is a pretty well-known problem with the movie, and people have tried to explain and make it work…even Zemekis.
I think I might have figured out a clean way to make the whole problem work without breaking anything in the movie (go with me on this, it'll take a minute to explain):
What if Biff handing himself the almanac in 1955 didn't just create a tangent timeline, but also a tangent Biff? At the moment of the handoff, two versions of Biff exist from that point forward. One version is the “normal” one that becomes Old Biff — the one whose life always played out without the almanac ever influencing anything. That Biff figured Old Biff was just some crazy old man, threw the almanac away, and forgot about it. He grows old, finds the DeLorean in 2015, steals the almanac (maybe realizing he screwed up when he was young and now he's determined to correct his earlier mistake), goes back, hands himself the thing, and then returns to his own 2015. So, in Old Biff's personal timeline, giving himself the almanac changed nothing. His life still led to the moment where he steals the Time Machine, and he returns to the future he came from – the one where he failed to make use of the almanac at all.
The other Biff — the tangent version — is the one whose life was altered by receiving the book. He’s the one who uses it, becomes a billionaire, and creates the hellscape Trump-inspired 1985. But that version of Biff isn't the Old Biff who stole the DeLorean in the first place.
So that's why Old Biff returns the DeLorean in 2015 in the same timeline he started in. He’s following his own personal history, not the global “new” timeline created by the almanac. He’s essentially anchored to the version of 2015 that produced him.
But…
Doc and Marty don’t travel along their personal timeline when they fly back to 1985. Time travelers in BTTF don’t always “follow their own timeline”; they travel to whatever version of a target time exists after a causal change. If they pick a different destination, they jump into whatever version of that time currently exists. So when Doc and Marty time-jump from 2015 back to 1985, they don’t land in the 1985 they personally remember — they land in the 1985 created by tangent Young Biff’s actions. Old Biff returned backward along his own personal timeline; Doc and Marty travel forward into a timeline that results from the changes made in 2955.
This is supported by Doc’s chalkboard speech, when he says “we cannot return to the future, because it no longer exists!” And it mirrors what happens in the first film, too: when Marty goes back to 1985, he ends up in the current version of 1985, not the one he grew up in.
ALSO, this makes Old Biff’s collapse after returning the DeLorean make more sense. He interacted with a past that wasn’t actually part of his own history. His body is basically stuck between two different sets of events and starts to fade like Marty did in the first movie. He returns the car, walks out, and dies because he’s coming apart under the strain of having just meddled in a timeline that invalidates his existence.
…I kinda feel like I'm missing the punchline here, but that's basically the idea. So far as I can tell this follows the movies' rules completely, so it all reconciles. But I'm still not 100% sure. Am I wrong?