Hi,
have you tried like below:
from data view drag and drop the field 'B' any where on to the layout and set the property as 'Hidden exclude from layout'. now in the hierarchy view place this new field exactly under the page 1 sub form
like; Page1(Sub form)
--> B(New field)
-->Header(Sub form)
-->Item(Subform)
now try the same way as I said earlier and this time Choose this new field from the path as page break condition. For me it worked many times. Just have a try and see.
Regards,
Satish