The built-in $VIMRUNTIME/syntax/json.vimuses the Vim hide function to hide quotation marks, supposedly to remove unnecessary clutter.
You must enable hiding by setting the option 'conceallevel'to 2or 3; defaults to 0(off). Similarly, you see quotation marks in visual mode due to the setting 'concealcursor'.
JSON , :
:verbose set conceallevel? concealcursor?
.