When using levels, curve etc within an action - the preview toggle only works once. The action has a pause so we can manually amend each adjustment. You are now unable to toggle preview on and off repeatedly. The only way to toggle between the two states is to change the settings within the control. It'll then toggle once again between states.
Photoshop 2015.1 (20151114.r.301 x64)
Brand new top spec PC i7, 32Gb ram, win10 et al