This issue comes up only under very specific conditions. The protagonist has to be a non-neutral human who is dualed to a cleric. While your original class is inactive, you are recognized as a pure cleric. This will trigger the Beholder Cult quest for one of the other temples. However, you still have to do the illithium quest for the Temple of Helm.
If you are following the fake illithium path, the quest can not be completed. After Sir Sarles has left, you are supposed to return to the temple with the illithium alloy and talk to Guardian Telwyn. The conversation requires Oisig to be present however, who isn't because you've done the beholder quest on behalf of another temple. As a result you are unable to initiate a conversation with Telwyn which is required to get your reward and finish the quest.
I've attached a save right before talking to Telwyn just in case.