Sans and Papyrus's relation to begin with is odd to say the least. Like what do we know about them? They're brothers, yeah, but individually. Looking beyond the surface level stuff, Sans is a judge, formerly a scientist of some sort, possibly worked with Alphys and even Dr. Gaster in the past, has a lot of mysterious belongings, powers, and is kind of obvious mystery bait.
What do we know about Papyrus?
...No, seriously, beyond the surface level, what do we know about Papyrus? Spaghetti isn't even his favorite food for crying out loud, and we only know that because Flowey of all people said so. While Sans baits mysteries with his backstory, the seeming lack of mystery bait on Papyrus's end has caused people to gloss over how much more mysterious than his brother he actually is.
Actually answering your question, sorry I kind of went off on a completely different tangent there, I'm not sure we can assume that Sans and Papyrus share the same abilities on the whole, considering what Sans has. While there's a line of dialogue implying Papyrus might also have access to Gaster Blasters, we have no hints of him having the i-frame removing power. So I don't think it's entirely safe to say that Sans has to remember because Papyrus does. And even then, Papyrus's memories of previous timelines aren't entirely clear either manifesting as dreams and deja vu, as you said.
And then there's the whole thing in the Asgore fight where you tell him how many times he's killed you and he just kind of shakes it off and I really don't know what to make of that, but that's another story entirely.
Man, I am really good at talking.>>110281234
God tell me about it. Alien X isn't even an instant win button, he's incredibly annoying to use for Ben. Hell, Feedback should be Ben's instant win button if anything, he might not be cosmic god tier but he's insanely overpowered without the drawbacks.