Cause of this issue (Pressure sensitivity loss on Mac): changing focus away from Photoshop and then back.
Workaround to avoid this or restore sensitivity without relaunch: If you switch focus back to PS by using the stylus to paint, you'll experience this. Instead, switch focus back to PS via CMD+Tab.
Also, if you run into this, using the eraser end of the stylus will restore pressure sensitivity.
Bug fix for this will be out in the next update, coming very soon!
... View more