Apply JSON Transformation Assertion is used to convert JSON to XML. If there is an object name called "content" it will not be created as a new tag when its being converted to XML.
See screenshot below which shows that content is missing.
This is a defect, but there are no plans to fix this as its a low priority. See defect DE367025
As a workaround, you could change the object name from content to anything else.
Example: Content (With upper case C)
Another workaround is place a regex before the conversion which changes the JSON element from content to Content, and then a regex after the conversion to switch the XML element from Content back to content.