I had the same issue, parsing the object went fine, but when I used to dynamic variable I got an empty result (which was not the case within the body of the JSON parse action).
Rather then using the value I tried to see the reference to the json object and I noticed that the path to the variable was incorrect.
When I use the dynamic variable within my expression I saw that it used
body('Folder_Information')?['ServerRelativeUrl'], but the result value of the JSON object was stored within a parent "d" element. So I changed my generated expression to
body('Folder_Information')?['d']?['ServerRelativeUrl']and my value is now available within the action where I want to use it!