Workflows that allow looping run server side, so they do not run as a specific user and thus cannot access user specific columns. Only the App Interaction workflow can access user specific columns because it runs client side as the user that is signed in. Unfortunately the App Interaction workflow does not allow looping through multiple rows. A bit of oversight on Glide’s part and a very frustrating limitation.
My best suggestion is a method that I use, which is a bit of smoke and mirrors, but very effective.