A word for this type of sequel
Solution 1:
It's a non-canonical sequel. While it's unusual to for things to be retroactively made non-canon, it happens, with a prominent example being Star Wars Legends:
Aside from film and television adaptations, which have been directly adapted into other mediums, such as novelizations, comics, and video games, the franchise has been expanded into original storylines. Except for direct adaptations of the films, only works released since 2014 are considered part of the canon (with the non-canonical works rebranded as Legends).
Star Wars expanded to other media