Is it set before or after either game? Timeline-wise, I'm wondering if the exact same characters could complete all three quests in the same timeline. CotM has to come first, since that's where they meet. Not sure if it's set before or after PoI, since no one calls you the heroes of the rift. It seems more likely to me that Lost Valley has to center around a different group of four characters on a parallel adventure for the timeline to work in one cohesive universe.
Pretty sure Lost Valley is supposed to be happening around the same time as Crown of the Magister, but it's probably intended to be done with different parties. Some of the banter you hear in Palace of Ice references some things they heard happened in the Lost Valley, so Palace of Ice happens after Lost Valley.
Galar Goldentongue is there in both. My assumption was that Lost Valley is with another party of adventurers happening around the same time that the Crown of the Magister party is in Copperann.
The timeline of Lost Valley is not explicitly stated, but it is generally believed to be set after the events of Crown of the Magister. This is because the characters in Lost Valley do not refer to the player characters from Crown of the Magister as the "heroes of the rift," which suggests that they have not yet completed that adventure.
The Lost Valley is set just after the events of Crown of the Magister though it may possibly start just before the final event the closing of the rift. Because the opening intro to Lost Valley goes "As the Principality of Masgarth prepares for war."
But Lost Valley is set before Palace of Ice because in Palace of Ice, Masgarth are no longer preparing for war they are at war.
Finally I agree with jannick the party is not the party that did Crown of the Magister simply because they are level 1 with none of the equipment collected in Crown of the Magister.
Both feature Galar Goldentongue. I figured that the Crown of the Magister group would be in Copperann at the same time as the Lost Valley adventures.
https://forums.solasta-game.com/forum/when-is-lost-backrooms-valley-set-compared-to-cotm-and-poi