Photoshop :: UNDO And BACKWARD Not Working For Brush Strokes?

After making three consecutive brush strokes in my image:Click on Undo once, History list backs up one step, but last stroke remains on screen. Press Undo again (Redo) and History list returns to last position, last stroke is still on screen. Click Undo again, History list backs up one step, but now brush stroke disappears as it should.
 
Or, after clicking Undo the first time, the brush stroke will disappear if I apply another stroke or enter a command, e.g. add new layer.
 
After making three consecutive brush strokes: Use Backwards command, the History list backs up one step, but brush stroke remains on screen. Use Backwards command a second time, History list backs up another step, the last two brush strokes disappear from screen. Use Backwards a third time, History list backs up another step (now 3 steps total), and the first brush stroke disappears as it should.
 
These commands work fine when creating three consecutive layers, then undoing or moving backwards.
 
I am using CS 5.1 for Mac running OS Lion 10.7.4. This problem occurs using either a trackball, Wacom Intuos4 tablet or keyboard shortcuts. Can this be a screen-draw bug in the brush tool?