It's just Mallow and Lana don't have much agency. They have quirks but none that really develop into proper flaws that could make for character development and they never really have agendas that are demonstrated enough that they put a whole lot of display in their niches. Even more episodic stuff they take part in like the class exercises or bad guy battles or UG missions or whatnot, they always do the least of or just win effortlessly with barely any method. They verge a little too much into blandly perfect territory, making them kind of boring in action outside of basic gags. I feel this is why we've only had one girl-centric episode and even that one was revolved mostly around just said gags, compared to how character and chemistry driven the ones revolved around the boys are.
The Tapu Fini episode was the nearest to the show TRYING to hone in on a proper fallible human side to them to develop from in my opinion but even then it was more an external force and concerned something when they were very young kids, and it didn't really click any long term role for them besides Mallow now perpetually glomping a useless hedgehog.
The others have flaws and agency to some degree, they have things they work or improve upon, or idiosyncrasies that liven up the plot or cause personal dilemmas, all things that can give them star appeal so they don't feel like ineffective props led by the plot. Lillie and Sophocles being brought into the league while seemingly actively avoiding giving them onscreen battle experience still bugs me as much as with Mallow and Lana, but unlike them, they at least had something else to round out their personalities throughout the series. There doesn't seem nearly as much averseness from the show into making them fallible fully realised characters outside that one element.
On a side note, this episode really makes me notice how annoying it is when the show keeps insisting on halting things to have ALL SIX CHARACTERS chime in a redundant uplifting phrase one by one.