I have a theory. So Asriel was originally a Boss Monster, right? And would therefore require a Boss Monster soul into order to have feelings, right?
So maybe, when he absorbed the monster souls, because he absorbed Asgore or Toriel's soul, that's why he could properly feel again. And it took a while for him to completly feel as he had before because the ridiculous amounts of LV he gained as Flowey twisting his morals?
Seeing as he had no monster soul as Photoshop Flowey and only human souls were absorbed that time, which don't inherently contain compassion and love and he displayed the same emotions as Flowey, I feel this proves my theory. What do you think?
But if this is true, you'd need to get either Toriel or Asgore's soul to allow Asriel to return to his former self and stay that way. Maybe that's why you can't save him.Â
Thanks for reading! Please give me feedback.