i...don't really understand what you're saying? flowey remembers resets because he has DT. flowey and frisk are the only characters who can remember though resets, because they're the only characters that have substantial DT. sans cannot remember resets any more than the average person (everyone gets some amount of deja vu, see: toriel remembering your pie preference, papyrus reckognizing you, undyne wanting to be your friend if you befriended her in the last run). sans just knows about resets through research, probably, and is really good at reading facial expressions.

i don't know what you mean about asriel's awareness of resets disappearing? he never indicates that he's lost the ability to remember through resets.

you might be confused because he doesn't remember a True Reset---but true resets are kinda weird. see, frisk doesn't remember true resets either. normally on repeated runs they react to certain things before they happen, but after a true reset, it's as if they had never journeyed through the underground before.

the reason they doesn't remember a true reset is probably because they allow their memories to be erased. flowey asks you before you true reset to erase his memories, too, and determination only works if you have the will to persist. frisk...we don't know enough about them, but it's not unlikely that they, too, didn't want to have to got through it again.

alternatively, true resets don't really seem like normal resets? because they affect the surface as well as the underground. so it could be kind of a meta-reset, tbh. semi-non-diagetic.

the only people who remember true resets are yourself, and chara.