Im sure Armin still cares about eren and still will be there when he needs him. I think his face has more to do with the disapointment/sadness Armin felt towards Eren for being suicidal and the background in the image of their joined hands symbolizes Armin getting Eren out of that dark place (even Levi gives a comment on Erens resigned face). And the look Armin throws Mikasa when levi kicks Eren says "he needs to be scolded"
btw, when I say being suicidal, i mean risking himself like that in that mission without telling them
I like your analysis and optimism better than my morbid one. Now that I look closely, his eyes look more sad than resentful. I'd rather believe too that the light in Armin's side is there to show he's rescuing Eren from the darkness than showing the separation and future conflict between them.
If we weren't sure about Armin and Eren might not becoming friends anymore before with Isayama's interview, it is much more apparent that is the case from their reunion here alone. See those eyes of Armin when he helps him up? Those eyes are of resentment and conflicted feelings. It seems Isayama has drawn not just his face, but his eyes up close to show that. Also, remember all those times when Armin lend a hand to help Eren while smiling? There's some kind of significant meaning here. He's not smiling when he's helping him up. In fact, remember in the past we would see light when their hands would join? Armin's side of the hand is light, while Eren's side shows darkness. This is to show there is no longer any spark of familiarity between them. It appears he is feeling some morose vindictiveness towards him. I think the image of Armin offering a hand is an allegorical meaning to "he's with him 100%." Something tells me that the next time Eren needs a helping hand, Armin won't offer it...
I'm quite surprised with Armin's persona. I expected him to be more cold-hearted with the whole "sacrifice everything for the cause no matter what" like Erwin. Grown up, he actually seems more like Jean, or at least emotional like Mikasa, caring about human lives, which is good to know he's not ruthless. I wonder if it's Bertolt's influence on him. I HOPE THEY STAY FRIENDS! I'D HATE THEM TO LEAVE THEIR SEPARATE WAYS!!