In the grand scheme of things, considering that a checkbox can only show a binary value, the "proper way" (In my opinion) to handle values in the checkbox, is not to allow focus (via .When) in the first place (in this case), based on the current context (which is apparently dictating what the value of the checkbox should be, and should set accordingly).
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This is incorrect, it is a 3 state value: NULL,.F. and .T.,
then it can to be ambiguous and require a Input choice to Validate.

I can allow NULL,.T. into a data context and .F.,.T. into other.

In either case, I don't see the problem.

This is the problem:
VFPT donnot see the problem.