So, if Monsters can't hold determination in their bodies, then what's the deal with Undyne? Not only can she hold her determination, but she used it in her No Mercy boss battle against Frisk/Player & only started to melt when her HP got to 0. And even then, she still turns to dust after melting.

And think about it; The true Lab monsters melted & fused together after they had "Fallen Down" & woken up. Undyne used Determination when she was still alive (Or at least still holding on to life).

What if: Determination has different effects on monsters who have "Fallen Down" & monsters who are still standing?

You can't kill the amalgamates, but you can still kill Undyne even in her Undying form, and both technically have Determination in their bodies. Also, Flowey only unlocked (And gained knowledge of) his Save & Load abilities after he tried to. . . ummm. Commit self termination (That's okay to say right)?

What if, before Flowey was created, Undyne could Save & Load, but never used it because she never died in the Underground therefore had no idea about this power. Until Flowey was made & she lost it, then Frisk/The Player came & Flowey lost his Save/Load Powers.

This is just speculation & theorizing (I have no idea if someone else in the fandom has talked about this before, i'm sorry if they did I just wanted to ramble about something I found interesting) But I do have faith in this somewhat. Feel free to tell me what you think about this in the comments section. Also, thanks for reading this far down.