There is a constraint on offset as follows:

      <xf:bind nodeset="offset" constraint=". >= 0" />
    

Try doing Decrement Offset and Submit and you will see that the constraint is violated on submission.

Try doing Just Decrement Offset followed by Just Submit and you will see that the constraint is preserved.

Offset:

Increment Offset and Submit Decrement Offset and Submit Just Submit Just Increment Offset Just Decrement Offset