I think you made a mistake. You need to insert a vertical bar before your variable. Like I did in this quick test.
I think this is properly explained in the documentation for the Prompt action....
If you want to provide a list in a variable, then you do so by starting the field with a bar (“|”), followed by your list (or your default value and then your list) in a variable (which contains choices separated by
|), for example “|%Variable%Current Choice%|%Variable%Choice List%”.