I think the button color changes in recent release introduced a bug.

The secondary button colors are hard coded in _variables.scss which prevents overrides via update of the matching variable. The other button versions correctly use variables like before but the secondary style is hard coded:

'primary': ( 'background-color': $primary, 'text-color': $white, 'shadow-color': $primary, ), 'secondary': ( 'background-color': #e3ebf7, 'text-color': #285192, 'shadow-color': #e3ebf7, ), 'success': ( 'background-color': $success, 'text-color': $white, 'shadow-color': $success, ),