System Information
Operating system: Linux-5.3.0-51-generic-x86_64-with-debian-buster-sid 64 Bits
Graphics card: GeForce GTX 1060 6GB/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 450.102.04

Blender Version
Broken: version: 2.92.0, branch: master, commit date: 2021-02-24 16:25, hash: rB02948a2cab44
Worked: Not sure, I just checked 2.83 LTS which shows the same behavior as 2.92.0

Short description of error
In Node Editors the "Select Box" tool ignores the select modes "extend" and "subtract" from the Active Tool settings (N panel). Instead, the current selection is removed on mousedown, which is the expected behavior of the first mode "Set a new selection". However, it works just fine for the Lasso and Circle select tools.

Exact steps for others to reproduce the error

• Add two nodes
• Use the Select Box tool to box select one of them
• Set the select mode to "Extend existing selection"
• Box select the second node

This will automatically deselect the first one.

Thanks for looking into this. :-)

EDIT:
Added a test file.