She still couldn't take over Sayla because she was too strong. So she took over just that ability. It made sense. There is precedence for this (taking over just a part), Elfman had done it to Rufus' arm beforeIf you can't consider Erza vs Kyouka legitimate. Might well as call Mira vs Seilah bullshit. She couldn't takeover Seilah but somehow after thinking about her friends, she miraculously pulls a take over from her ass.
Even if it doesn't make sense to you, it's still nowhere near as bad as Erza vs Kyouka. That shit is the most illogical stuff I've seen in FT, that's an accomplishment considering the sheer amount of asspulls and bullshits present in this series lmao