Steps to recreate:
1. print screen > capture the screen shot
2. open image editor
3. add high light
4. click on the outlook button and add it to outlook window. (In my case I added it to an already open window the first time as well.)
5. Now take a new print screen
6. open image editor
7. add high light
8. click on outlook and add it to the previous open window
9. The previous image gets replaced.
10. Repeat the steps 1 to 4 and the image gets replaced again.
It doesn't add the new image to the list like word or powerpoint.
I am currently using greenshot version 18.104.22.168.