Why do some submissions get AC in Original constraints but WA in subtasks.

I have seen quite a few no of problems having submissions which have passed original constraints but not the subtasks ( like this ) .
I think something needs to be done about such submissions since I am pretty sure the authors would want such subs to give WA. I suggest codechef should enable a feature which gives points for original constraints only if all subtasks get AC or make the test cases stronger (ahem ahem) ,

Another example : https://www.codechef.com/viewsolution/20381093 .

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It could happen just because of boundary cases