Perspective has a vanishing point. Your second screen shot is directly in line with the cube, and hence you only see the front face. Ctrl 8 or 2 (on numeric keypad) will show this. Ortho (5 turns on off) is the view to use for plan view. (Hitting period "." on numeric keypad with object selected will give you direct view for axis aligned objects)
– batFINGEROct 10 '17 at 11:24
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Related https://blender.stackexchange.com/questions/86969/how-do-i-change-the-mode-of-perspective, https://blender.stackexchange.com/questions/13387/how-do-i-switch-auto-perspective-on
– Mr ZakOct 10 '17 at 11:37