Repeated the same steps with an input element, with exactly the same result. The attribute names cannot be anything which is not in the list. But otherwise everything worked, also changing attribute and value. In fact, if I want to, I can add several attributes to the same element. Also, I can delete attributes, and even the whole input element.
You must be using another program....
Open my RFF file attached, it has the top input element stuck with a placeholder attribute set.
But to repeat do this:
1) Open blank
2) First column drop a form component
3) In form component drop an input element
4) Select input element and expand the ATTRIBUTES PANEL
5) Click on Add "Select an attribute"
6) Choose the first one "Custom"
7) Change the name to "Placeholder" and value to "What I Like" (bear in mind this is a custom element, so naturally you can customise the name and value)
8) Now you will either be greeted with an error "The name is reserved" (Thats because you didn't type it exactly as above, CAPITAL P!)
The placeholder is set, click a different element and then reselect the input element you created the attribute on and voila, its set but you cannot edit it now.