Oh, I think, in all honesty, spanners are pretty much a cast iron certainty. For a start, we've not seen him actually change his number yet, he's just made the decision, as far as I recall. I'm not suggesting he's not going to, but my point us, we haven't yet seen Charlotte's reaction.
How likely is it that, given their history, she's going to realize he has changed his number and think "Oh, guess it's over, I'll concentrate on the kids and maybe set up a crochet Insta account"?
And given that she is Robin's biggest hang up, I would be astonished if Rowling doesn't decide to give us fireworks. I may have written a couple of fanfics that have them addressing and resolving the Charlotte thing, but a 10,000 word fanfic, and a whole book that has no doubt been planned with meticulously fiendish detail, are two very different animals.
Plus, you know, crimesolving to squeeze in between all the sexual tension.