I mean, I couldn't find any documentation or any example to compare with, but any expression I put in the box is not processed when accessing worklog data. If I put, for example, "this" keywork, in the column I can read the Json of the current object, but there I cannot extract any fields value if I refer to any property.
My intent was to extract the "worklog.updated" field in the report, but every attempt result in empty cells
Multiple fixes has been done in expressions and now documentation is also available online explaining how to use expressions. Hence closing this issue.