System Information
Operating system: Windows 10 x64
Graphics card: Nvidia GTX 1060

Blender Version
Broken: blender-2.80-9a8b6d51c12-win64

Short description of error
Box selection + shift(ctrl) key should function for Add, Subtract.
It is normal when a fresh start.
If you click 'Add' button, shift(ctrl) key function will not work anymore.

Exact steps for others to reproduce the error
New project.
Ensure you're in the Select Box and New button.
Enter Edit mode.
Test Shift or Ctrl keys to working add or subtract function.
Click Add button.
The Box selection only works for Add as expected but weirdly you can't subtract selection with Ctrl key.
Click the New button.
Now, The Shift or Ctrl key both will not work.