Was caused by event emulation (numpad and mmb) happening separately at each level of the keymap, so brush selection was latching on the event before it had time to test emulated values with view navigation entries (at a lower level in the hierarchy).
After discussion with Matt, decided that event emulation should be done at the root of the event loop, replacing the event with the emulated values. This is much more predictable and more logical as far as event emulation goes.
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