Marvel Studios makes it look easy, but keeping track of the continuity between dozens of films is understandably hard. Even though X-Men arguably sired the modern superhero movie, its sense of continuity is, well, pretty effed up. Like why is Hank McCoy seen as a regular looking human in X2 but then he's full on Beast-mode in X-Men: The Last Stand? Even though all these errors apparently got reset after X-Men: Days of Future Past, Fox is still making mistakes. Just look at Caliban, who shows up in X-Men: Apocalypse and then in Logan as almost a different person. According to James Mangold, there's a reason for that:
It's a funny, messy story of how so often these things are not as coordinated as everyone thinks. I actually had written him into our movie, and they didn't know [he was] in Apocalypse, and then they kind of wrote it in their movie, and they cast someone in their movie and I had not seen it and was working away on mine.
If you thought that the X-Men movies were finally going to get their shit together after Days of Future Past, then let me present Case A: Caliban. The albino mutant first appeared in the 80's set X-Men: Apocalypse (played by Tómas Lemarquis), using his mutant tracking ability to help both Mystique and Apocalypse. Then he appears as a supporting character in Logan played by Stephen Merchant, and he's basically an entirely different character, complete with a different accent. The folks over at Nerdist asked Logan director James Mangold about this snafu, and he admits that's it was a product of confusion and just generally no one keeping tabs on things.
Essentially what happened was that Apocalypse was in production when Mangold was at least in the scripting stage for Logan, so he understandably didn't know what was happening in that movie. And he didn't need to either because Logan is set in 2024 and is fairly separate from the rest of the goings-on of the X-Universe. It's pure coincidence that both movies ended up using the same somewhat obscure character, and when Mangold found out, he refused to find a different character due to liking the "particular energy that we wrote."
James Mangold's uncompromising vision obviously played out for the better with delivering one of the best X-Men (and maybe even superhero) movies ever, but it's still a little frustrating that no one at Fox seems to be on the look out for errors like this. It only adds to the argument that they should ditch plans for a shared universe and just focus on making great standalone films where continuity doesn't really matter. Deadpool, Logan, and even Legion all benefitted from that approach, and it wouldn't hurt Fox to stand out from the pack.
You can see Stephen Merchant's version of Caliban be really sassy to Wolverine in Logan, which is out in theaters right now.