A Goofy Movie/WMG

Roxanne and Max are still together, but at separate universities.

 * With all of the trouble the writers put Max through in the first movie so Roxanne would even consider dating him, they probably didn't want to cheaply undo that just for the sequel. That's why they had P.J. hook up with the beatnik and Goofy hook up with the librarian instead.
 * But in "Mickey's Twice Upon a Christmas" Max comes home from college with his girlfriend, Mona.
 * This makes sense, especially since Roxanne does make an appearance on House of Mouse on a date with Max, seen here.
 * That's because Max is still in high school in House of Mouse.
 * I submit that Max(, Donald's nephew and anyone under the proper age grade) has some involuntary time travel powers considering how they handle them throughout the various universes.
 * I'd like to think that maybe they did date for a while in high school, realized they weren't meant for each other, but remained friends anyway and still hang out during breaks from college. Boring yes, but such is life. It would be nice for Disney to do a friendship like that without pairing everyone with just one love interest.
 * That'd make perfect sense. I mean, obviously if they were still dating, there's no reason why Max would be flirting with the attractive cashier midway through the movie. Max is actually visibly attracted to many beautiful women in the entire film, throwing out the possibility of him and Roxanne dating out the window.

Peg and Pistol died in a car accident between the Goof Troop and A Goofy Movie
That's why Pete is much more of a Jerkass in the movie, especially to PJ. He's been going through a rough patch after his wife and daughter died, and is just trying to take it out on the people closest to him.
 * Replace dying with a divorce/separation and it works as well.

Roxanne dumped Max (or vice versa)
That explains Mona and her lack of appearing beyond House of Mouse. Not soon after that appearance, they broke up. It's not like high school sweethearts always stay together anyway; very few do, you move on to new people.

Pete and Peg got divorced
Peg got fed up with Pete's money-grubbing schemes and demanded a divorce, taking Pistol with her and leaving PJ with Pete. Most likely why Pete's Jerkass meter went up in this movie.

Stacey's popularity
I've come up with a number of explanations for why Stacey is popular despite her geeky appearance:
 * She throws awesome parties (the most likely explanation)
 * Her being friends with Roxanne elevates her coolness level (though we never get any indication that Roxanne is popular)
 * Max makes her look much cooler by comparison (the funniest explanation)

Mickey's Twice Upon a Christmas takes place in a different continuity from these movies
Those movies don't look Goof Troop canon so why would Roxanne exist and/or be with Max?