Thanks VikingOSX, the remove, then add procedure you described solved the problem.
Spoke a little too fast. The procedure works in the sense that the example in the Automator action panel displays correctly.
I still encountered an issue in that if I wanted to append <space character> followed by the formatted date—transforming "name" into "name 20210816"—I could not accomplish this by entering a space followed by the variable in the Add: box text field. The Automator example displays properly but the net result (of applying the action) still contains the UID and not the correct date string. So there seems to be a problem with a space character being the first character in the Add: box text field (although a string literal with a leading space, e.g., " hello" does work).
Played around with it and figured out that a space character can be the last character. So a workaround is to first add a <space character> to the file name, then add the formatted date (see below). Maybe a text box cannot contain a leading space before a variable by design but without documentation it's hard to tell whether this is documented behavior or a bug.
