This problem started after an update some time ago. When I first had WA the sim could do this section easily.
Ok, this is not for anyone doing challenges, but, otherwise, this is my work-round. First enter; Testingcheatsenabled true. Then enter; enablelotlocking true. When your sim is has broken the rock and can enter the area, save the game in case you make a mistake. Right click on the floor and enter build mode, Carefully remove the section of wall, including the door which cause the block. Leave build mode and continue as usual. This is a permanent change which allows re-entry at any time without further cheating and is useful if you want to return to raid the tomb on subsequent visits. I don't consider this to be cheating as the area is not meant to be blocked. Don't remove any other walls as you could destroy lighting and spawning points.