Reproduction steps:
1. Create a new project and build for iOS
2. Wait for Unity to finish building
3. Build again in the same folder
4. Unity will prompt you to either Append the existing build or completely Replace it
5. Click Append

Expected result: Unity will start appending the build
Actual result: another pop-up window shows up asking if you want to replace the build (read note 2)

Reproducible with: 2020.2.0a17, 2020.2.0a18
Not reproducible with: 2018.4.25f1, 2019.4.5f1, 2020.1.0b16, 2020.2.0a16

Notes:
1. A video of the issue is attached (2020-07-22 14-58-58.mp4)
2. In the background, the build isn't actually replaced but the additional pop-up makes it very confusing
3. I am not sure if any other platform supports existing build appending