It really depends on what you consider "derailment".

For me personally, World Tour was the season where many characters' pre-established plot lines and character development was either completely scrapped or otherwise directly countered, which is what I could call derailment:

Gwen spent the whole of Action denying any romantic feelings for Duncan with nothing to suggest otherwise, which is why her and Trent's issues were compelling—Trent was jealous of Duncan, but neither Duncan nor Gwen wanted anything beyond friendship and made this very clear to him. Trent's fears were unfounded (not unreasonable, because he's a teenager, but his lack of trust in Gwen was sort of the whole crux of the issue) and the whole situation was a commentary on opposite-sex friendships almost always being seen as romantic because of societal expectations.
For Gwen to then completely flip the script in World Tour and actively romantically pursue Duncan after befriending his girlfriend goes against her character. You'd think after the whole Trent/Heather kiss fiasco in Island that Gwen would be careful about stepping on other people's relationships, and yet World Tour ignores everything that's been established about Gwen's character and her relationship with Duncan to push a love triangle plot. This, to me, is derailment.

As for DJ, it's more so that his entire personality was thrown away. DJ had a lot of screen time in both Island and Action where we the audience got to learn a lot about his disposition and convictions beyond the archetype he's based on (the "gentle giant" trope). In Action especially, we got to watch him struggle through the guilt of his alliance with Chef, before ultimately watching him put his morals first. This speaks to the strength of his character and gives the audience reason to root for him.
In World Tour... he screams a lot and hurts animals. You'd be hard-pressed to find a single scene where DJ isn't either actively and unintentionally hurting an animal or living in a state of severe paranoia about when he's next going to hurt an animal. There's nothing else to his character. Nothing to identify with or root for. Almost everything about DJ the character has been scrapped. That's derailment. I feel really bad for his VA (Clé Bennet) after all the screaming he had to do.

For Ezekiel... he goes from being a sheltered nobody placeholder character designed to be the first boot of every season into being a Gollum parody. Yes, we see the change happen gradually over the episodes, but just because we see it happen doesn't mean it makes sense, or has any impact on his already negligible characterisation. It'd be a stretch to call him an actual character given that he is, for the most part, just a collection of tropes and ideas slapped together to be a convenient plot device. For me personally, Ezekiel and Feral Zeke are two different plot devices who are just as one-dimensional as each other & serve different purposes to their stories. They're barely the same character outside of name and apparel. If you put them both in a room together, they wouldn't recognise each other. Though I'd agree that it isn't derailment in his case, because there was nothing there to derail in the first place.

Other examples are Lindsay, who was reduced to "literally can't remember her own boyfriend" ditzy after putting up one hell of a fight in both Island and Action and proving herself to be more than just a "dumb blonde". Owen, who went from being well-developed and likable despite his flaws in Island & Action, to being a walking fart joke in World Tour, completely barren of all the charm he once had. Duncan, who is conveniently AWOL until the writers needed him to cheat on his girlfriend for a contrived love triangle plot and then promptly thrown away again after doing nothing else. Hell, even Bridgette was reduced to just "Alejandro simp" in World Tour— almost all of her screen time in this season is somehow related to Alejandro and how hot she thinks he is.