Changes to Trendline Labels not being saved

I want to add some text to trendline labels. The label is originally added as R-Squared value. I changed the category to text and added my text. This works OK but after saving and re-opening, the added text has gone. I also tried copying the chart and pasting as an image but again the added text has gone in the resulting image. Is there a way to add text to a trendline label so that it stays there?

trendline_label.png