The issue can be solved rather easily: Instead of pressing R1 to transfer the thrall to a workbench (he would wind up in the workbench’s inventory) press X on the thrall, then move the highlight to the slot for the thrall and press X again.
The thrall is placed there without any detour, and the last step unlocks as it is supposed to.

Looks like the expected behavior of R1 has been lost in one of the previous updates. Could it be that the event handler for pressing R1 has been messed up so that the transfer isn’t initiated correctly any more?