A single file can't change its status between staged and unstaged but select all function works.
The demo video of bug reproduction provided here: https://drive.google.com/file/d/13-2kl2kYSzzmZqsbzQuZJFxauluBQR0Z/view?usp=sharing
Restart Sourcetree, the computer didn't work.
It has been a long while, I thought SourceTree will release a new version to fix the bug.
SourceTree version: 4.0.1(234)
macOS: Catalina 10.15.3