Thanks for re-testing! Found it: the glowing quest bubble in the Dark Lab was floating a little below the cabinet's real (invisible) spot, so clicking the bubble did nothing. The bubble now sits right on the cabinet, and clicking it gives you the Secret Document. This will be in the next update (not live yet). Please walk to the right-hand ledge in the Dark Lab, click the bubble, and check the document lands in your Etc tab.

Marking this fixed. The change ships in the next update. If it still happens after that, please reopen this report.