I have noticed a fun thing, Sampo is always there or relevent at the beginning of a new big patch.
1.0: First person meet on a brand new planet, helped us subtly till the end only to leave us without explanations or goodbyes. (if you don't count the trailblazer's dream where he breaks the 4th wall, wich looks like a message mostly aimed at the player than the MC.)
2.0: Penacony, even if it's Sparkle disguised, the real Sampo was still relevant in Sparkle's companion quest. We also got Tatalov's lore that could potentially foreshadow's Sampo's past.
3.0: Of course they wouldn't put Sampo in Amphoreus out of nowhere but despite that, there's an NPC, I would assume, entirely referencing Sampo. From his hair color, eyes color to his mannerism and speech pattern even the trailblazer acknowledged. (there's also a few of references to Sampo on Amphoreus)
4.0: Sampo makes a short appearance in Planarcadia but doesn't stay relevant after this so far. It looks like he was just there to get his mask back and that's it. Maybe he'll be relevent again later this 4.X patch or muuuuuch later.
It just look like there's a pattern where Sampo appears briefly at the beginning of a patch in the main story but never stays relevant. Kinda similar to his first apperence in Jarilo; he keeps on appearing and disappearing. He gets a few appearances (if lucky) in events and small quests but nothing much.
I don't really know what to make of this information or if it really matter but I like the fact he's always there no matter where we are, lurking around as if making his presence known without interfering (yet)