Plot hole in noticed.
In the season 3 finale, Zaheer tells Korra that she cannot stop the poison unless she somehow manages to learn how to bend platinum. Suyin Beifong then goes on to bend the platinum poison out of Korra. Which is fine, Suyin is a metal bending master!
But then in the season 4 finale, when both Lin and Suyin are on the huge mecha suit, they are unable to open the metal up because the outer layer of the suit was made of platinum. Also, earlier in season 4, Korra bends all the remaining platinum poison out of herself.
So we know that at the very least Togh, Korra and Suyin are able to bend platinum. Whilst it was never showed, it would be safe to assume that Lin would also be able to bend platinum.
Yet in the season 4 finale, when they are trying to breach the mecha suit, no one seems to be able to bend platinum. But then when Korra and Kuvira are fighting in the mecha suit cockpit, both are able to bend platinum again.
Is there any explanation for this plot hole? Or where the writers of the show lazy?
PS. I love both LTAB and TLOK. So this isn't an attack on the franchise, just something I noticed