You have to get close to the rock to get the cutscene. Head east after passing through the stone archway and you should find it. If you have reached the correct rock without being able to move it, see below.



I think this was fixed in the patch, but there was a possible bug with finding the cure ingredients and moving them in your inventory before talking to the alchemist. That could trigger a quest log update prematurely, and prevent the cut scene at the rock covering the mushroom cave. There are a couple workarounds for this, however:

either trigger the cutscene;
[*] Complete all other quests and save the game.
[*] If you have a save before finding the alchemist's ingredients and moving them, copy the 2 story.* files from that save into your latest save, otherwise you can use these files from a save at the beginning of the imp village.
[*] Talk to Altoflix in the north of the village (if required), then the chieftain's son to get the quest to find the alchemist, talk to the alchemist once (only) and head to the cave.
[*] Continue with the game.

or

bypass the rock to get directly into the cave;
Teleporting anywhere (via hex editor)

Complete all other quests before giving the ingredients to the alchemist.