I can see the logic of combining Instants and Sorceries, and even Artifacts and Enchantments to a lesser degree, though I'm not sure if mixing types at all is intended behavior or not. Planeswalkers may make more sense being grouped alone (or with creatures, if types are intended to be mixed).
Under 'Deck Sort Order,' when choosing to sort by type, cards are sorted as follows:
Lands, Creatures, Instants / Sorceries (mixed), Artifacts / Enchantments / Planeswalkers (mixed)
I can see the logic of combining Instants and Sorceries, and even Artifacts and Enchantments to a lesser degree, though I'm not sure if mixing types at all is intended behavior or not. Planeswalkers may make more sense being grouped alone (or with creatures, if types are intended to be mixed).