And the sequels had significant backstory that was not in the original or in the backstory to it -- events that happened after the original book and before this one.
I don't think I approve.
Well, as a general rule. Some events happening between stories are logical. Particularly if there are not the same sort of things. If the hero tracked down the MacGuffin with the aid of an apprentice wizard whose master died, it's only logical for the wizard to reappear, no longer an apprentice. If the hero and heroine ran around killing goblins and then married in the last chapter, having a few diminutive versions running around underfoot is only logical after some years.
But to introduce, after six years, another character that they went goblin-killing with in the last few years comes across as odd. Or if they were thinking of marrying, and when the book opens, they had broken up, and the hero had found someone new.
Part of it is that we are told, not shown, the events. This can easily lack conviction. They were starry-eyed about each other, and we saw that in vivid color. To fall apart like that is unconvincing. Especially how on earth did he hitch up with her when we know he can appreciate the heroine? Doesn't really work.