see the other bug report for the additional bug that occurred on the same move: 1) game not ending, and 2) only one sarcophagus claimed when 2 adventurers advance simultaneously into the tomb

The rule says on page 2: "Next, the player performs the action shown on the tile upon which the adventurer ended its movement." The text on page 5 confirms that the only actions existing in this game are the actions of the tiles.
The tomb chamber has no tiles, only two sarcophagi. Therefore, you cannot say that looting a sarcophagus is an action, on the pretext of a so-called discussion whose existence or content cannot be verified.

You must consider the theoretical (though unlikely) situation where all adventurers of a player would enter the tomb chamber. Considering your answer, only one sarcophagus could be recovered at this time, and I presume that another adventurer present in the burial chamber will not be able to recover the second sarcophagus on the next turn. So, for this only player, the game would be over. With such a rule, it seems obvious to me that the use of keys should be optional, as long as the player can move at least one adventurer.

I don’t want to sound unpleasant, but the fact is that your connection is generating an error (BGA service error), and I don’t have access to the linked discussion you’re talking about, even having been part of the reviewers. I think it’s the same for other BGA members.

It seems to me that an important rule change such as deciding that the recovery of a sarcophagus is an action, and in addition a mandatory action, should be set out on a page accessible to all.
The result of such a change is that a second investigator entering the tomb chamber during the same player's turn does not recover a sarcophagus. This disadvantage could be avoided if the player was not ever required to use a key available to enter the tomb chamber (unless this is the only possible move) or if the use of the key was considered as part of the action (so only one key could be used during a player’s turn).