After setting a B&W Mix, pressing the v-key returns to the Color Mode, as expected. Pressing the v-key again returns to the B&W Mix with the B&W Mix sliders all returned to 0 and the photo returns to the original state before the B&W Mix was set.
This does not occur when clicking Color and Black & White in the Basic Panel. The B&W Mix Sliders stay as they were set.
It seems that the v-key has become dysfunctional, or am I missing something?