Bug - numbers on keypad disappear – Math Fact Master Review

This has a strange bug. When you are doing a drill, after the first time you click a number on the keypad, that number then disappears and the square on the keypad becomes blank. Unless you know the keypad by heart, which my eight-year-old doesn't, then you're out of luck with this app. I feel cheated.
Review by RG-bk on Math Fact Master.