After the new August 2019 update...
I have a report page which has 50 objects (matrixs) I used the new feature of grouping an object. I managed to group those matrixs into 25 objects. When the subscription email was being triggered it was sending out an incomplete png of the report page.
I ungrouped all those objects and everything was working fine. It seems that grouping objects/visuals increases the render time.