After the below attempts I finally got this to unlock.
These attempts caused the challenge to fail;
With the challenge included with other challenges;
- A mix of Medium / Large artefacts with both players collecting them, totaling more than 5, but neither over 5, with small artefacts in the mix, did not work.
When Adventure Besties was the only selected challenge;
- 5 Large artefacts collected only by the host, with no medium and small artefacts, does not unlock this.
- 3 Large and 2 Medium, collected by the host with no small artefacts, while the other player went off to look for the next crypt, did not work.
Solution
On the last version of the game v1.1027.0.0 on 2022-10-31, What finally got the achievement to unlock for us was the following setup:
- Adventure Besties was the only selected challenge, chosen by the host (me).
- Cards selected were Toxicity, Eagle Eye and Tomb Raider
- 0 Small, 5 Medium, 5 Large artefacts were collected, so 10 total.
- We did not keep an exact tally of which player collected how many of each artifact, but we're sure that I only collected 4 artefacts while the other player collected 6.
- Both players split up in the map from the start, so we were not in the same crypt or anywhere near each other when collecting the artefacts, breaking the theory that the host must collect the artefacts.
- The host lit the signal fire after collecting the artefacts. We managed to have it all done by Day 4.
We have a couple of theories as to why this unlocked when the others didn't, though by this point we were just fed up and ready to uninstall the game. Some theories include:
- Using the Tomb Raider card causes the Medium Artefacts to 'not count' since the skills don't get collected, and therefore breaking the possibility of being able to complete this challenge (I don't believe this to be the case as I'm quite sure someone would have discovered this after all these years)
- Despite the Large Artefact counter going up on the stats screen, the bow you unlock from the 4 bow parts only counts as 1 Large Artefact (this is more likely).
- It could just be buggy as **** and just a case of getting a lucky dice roll. Some ideas about things that cause it to break include having other challenges active or having another challenge active which involves escaping on Day 5.
All I can say, is that if you replicate the above setup that worked for me, then I don't see why it shouldn't work for you as well - and if it doesn't, then I guess it is just buggy and you have to keep trying. I do not believe a save deletion is needed, although at this time of writing I don't even think it's possible to delete the cloud save from a Windows 10 game...
Good luck!