The line is still there, it's just not as big and bold as in the first image. The "No Measure Value" gray is what's   The running sum of sales is a much higher value than the other running sums, so it flattens the others out quite a bit. This makes their confidence bands more narrow (since the trend is much more predictable and therefore can afford a narrower confidence band), and that might be why it looks a bit different in the second image. You could change the color being assigned to your 45 degree line to something a bit more offset, to make it stand out more.

Here's where the weirdness comes into play. The trend line is there, but you can't see it unless you select some data points:

In the Trend Lines Options, you have selected "Show recalculated line for highlighted or selected data points". I think what's happening is that when no data points are selected, there technically is no trend - it's a constant ratio of x-value to y-value. When you select data points, however, it forces a calculation. This could explain why the trend line ONLY shows up when you select data points. I know it doesn't answer how to get a 45 degree reference line on there, but it's a step in that direction!

I'm experiencing a somewhat similar scenario where adding a running sum causes changes to my reference line.  Here's to hoping for a solution.

Window Sum calc causes change in Reference Line