Atsuko was being rational about things. She wasn’t sacrificing her own well-being against this freezing enemy known only as ‘the cold,’ she was taking her time to ensure the thin ice doesn’t collapse beneath her… Sure, there may have been something in there, but Atsuko was convinced she would die the second she touched the water. So she was being careful.
Matsu was not. In fact, they were not only upright, but sprinting.

“J-Just what are you doing, y-you’ll break the—”
Atsuko instinctively began moving backward, only caring to protect her own life from being extinguished by the polar waters below. And surely, the other two would fall in, and that perfectly good scarf would go with them. She braced herself for it to happen…
But it never did, and Matsu landed unceremoniously on the ice next to Haruka. She gave away her own scarf, something Atsuko could not fathom or understand.
Once she was sure the ice wasn’t going to be making any surprise breaks after Matsu’s run, Atsuko returned to slowly crawling toward the other two. And the scarf.

“…Don’t… d-do something like that again. I-It’s… foolish.” Atsuko did not clarify which of the two she was speaking to, and truly, she was speaking to both of them. Don’t sacrifice your own warmth to Haruka, and don’t put me in danger by doing something so stupid to Matsu.
Soon enough, the lone scarf on the ice was finally in Atsuko’s gloved hands. It wasn’t warm on its own any longer, but still, she pulled up her own scarf and put the second scarf on over it to add an extra layer against the cold. Anything to make this frozen nightmare less unbearable.
Still no ‘Thank you’ from her to Haruka. I’m so sorry.