This is a side effect of "Configuration: Cropping". This configuration works by measuring the text width and determining line breaks and text cropping (the "..." ending) according to label width (which equals the node width in your case). However, due to features such as sub-pixel rendering and text anti-aliasing, text width may differ between rendering on screen and exporting to PNG (or any other export format) and therefore line breaks and text cropping may differ, too.
E.g. suppose you have a label width of 100 and default insets (meaning a margin of 2 on each side). Suppose further the first 5 words of the label text have a width of 95 on screen but a width of 97 when exporting to PNG. Since the available space has a width of 96 (label width - left inset - right inset), all 5 words will appear in one line on screen but will be split over two lines when exporting.
Since it is technically not possible to do sub-pixel rendering and some forms of text anti-aliasing in a raster image such as PNG or JPG, there is not much that we can do to change/improve the situation.
You will need to enlarge your nodes manually such that the text also fits inside the node in the exported image.