I basically followed this YouTube tutorial's idea of doubly-inverted rectangles to highlight text, but I used a "highlighter" effect because I liked how it just changed the colour of the text, rather than the background. In the Fusion tab, I've got it looking great:

But in any other tab, and most importantly when exporting the video, it looks completely wrong:

My Fusion nodes are as this:

The three nodes left and right of Merge2 are mirrored, for the 1st and 2nd line of the highlighted text. The chain goes:

1. Rectangle with "Invert" animated to move over to animate on the highlight.
2. Rectangle with "Invert" and Paint Mode "Merge" that statically stays over the text to be highlighted.
3. Background with "Apply Mask Inverted".
4. The Merge node.

So two questions:

1. Why does the output look so completely different in Fusion vs Edit/Deliver tab?
2. How can I get around this to achieve the desired outcome (the first gif)?