Apparently, when you first turn the Flex button on it also checks the Enable Flex checkbox and enables the Flex feature, but after that the Flex button only turns the Flex waveform display on or off, not the Flex feature itself. Once it's enabled, it seems that clicking the Enable Flex checkbox disables Flex, and hides the waveform Flex view and turns off the Flex button. Not the most logical system, but that seems to be how it works, at least here.
Sometimes when you're in the Region view and you enable or disable Flex it switches to the Tracks view controls, but the Track tab does not light up as it should -- a graphic bug, apparently. If that happens you can simply click the Region tab to restore the Region controls.
And again, that's separate from Pitch Correction -- pitch correction is not controlled by the Flex button, only by the Pitch Correction slider.
Here, the Pitch Correction slider and the buttons below (under the Track tab) and the Time Quantize controls (under the Region tab) are always visible (in their respective tabs, except for the aforementioned graphics bug).