Isn't a bug.
Normally you map a logical field to an Checkbox, or a Numeric field.
For Logical field, set the Checkbox value to .F. on the Form designer, this is because the original Value of a Checkbox is 0.
If you are using it with a View, a cursor or a table than accepts NULL values, this is the correct behaviour, try to check the values to ensure isn't a NULL.
Hope this help
Mauricio
Be fast like the Fox
Be free like the Penguin
Se Rápido como el Zorro
Se libre como el Pingüino